In the 20th century, after their doctorate, German scholars who wish to go into academia usually work toward a Habilitation by writing a second thesis, known as the Habilitationsschrift. This is often accomplished while employed as a Wissenschaftlicher Mitarbeiter or Wissenschaftlicher Assistent ("scientific assistant", C1) or a non-tenured position as Akademischer Rat ("assistant professor/lecturer", both 3+3 years teaching and research positions). Once they pass their Habilitation, they can work as Privatdozent and are eligible for a call to a chair.
Since 2002 many paths may lead to a full professorship. One can reach a professorship at a university by habilitation, a successful evaluation as a junior professorship (after 2-3 years) or equivalent performance. In engineering this is often attained through expert knowledge in the industry, and in natural science often by the number and quality of publications. While universities and Fachhochschulen (Universities of Applied Sciences) do not have the same legal status, there are no formal differences in academic ranks. Since a new salary scheme was introduced in 2005, both types of universities can appoint W2 as well as W3 professors. In general, a professor at a Fachhochschule has not gone through the process of habilitation or junior professorship. Instead, a doctorate and at least three years of work experience in research and development outside academia are required. Usually a professor at a university of applied science is more focused on teaching while a professor at a traditional university is more focused on research.
It is worth noting that in Germany it has been debated whether Professor is a title that one may retain for life once it has been conferred (similar to the doctorate), or whether it is linked to an office and ceases to belong to the holder once the professor quits or retires (except in the usual case of becoming Professor emeritus). The latter view has won the day -- although in many German states, there is a minimum requirement of five years of service before "Professor" may be used as a title--and is by now both the law and majority opinion.
Main positions
Professor (Prof.): Since about 2002 the standard title for full professors at universities in Germany.
Professor ordinarius (ordentlicher Professor, o. Prof., Univ. Prof.): professor with chair, representing the branch of science in question. In Germany, it's common to call these positions in colloquial use "C4" professorships, due to the name of the respective entry in the official salary table for Beamte (civil servant). (Following recent reforms of the salary system at universities, you might now find the denomination "W3 professor."). Today in most German federal states this title is obsolete for restaffing. Since 2002 all full professors at universities and applied universities are called "professor". In some federal states like Baden-Württemberg it is still possible for a professor at a university to make application for the title "Univ. Prof." under special conditions.
Professor extraordinarius ("extraordinary professor", außerordentlicher Professor, ao. Prof.): professor without chair, often in a side-area (comparable with Associate Professor in other systems), or being subordinated to a professor with chair. In Germany, it's common to call these positions in colloquial use "C3" professorships, due to the name of the respective entry in the official salary table for Beamte (civil servant). (Following recent reforms of the salary system at universities, you might now find the denomination "W2 professor" or "W3 professor without chair-function" in the state of Baden-Württemberg). Often, successful but junior researchers will first get a position as an ao. Prof. and then later try to find employment as an o. Prof. at another university. In Prussia before the First World War, the average salary of a full professor ("Ordinarius") was double that of an associate professor ("Extraordinarius") and up to nine times that of a professor starting his career.
Professor emeritus (Prof. em.): just like in North America (see above); used both for the ordinarius and for the extraordinarius, although strictly speaking only the former is entitled to be addressed in this way. Although retired and being paid a pension instead of a salary, a Prof. em. may still teach, give exams and often still have an office.
Junior-Professor (Jun.-Prof.): this position started in 2002 in Germany, this is a 6-year time-limited professorship for inexperienced young scholars without Habilitation. It is supposed to rejuvenate those who are eventually supposed to become professors ordinarius in other institutions. The concept is intensely debated due to a lack of experience with this new approach. The main criticism is that Juniorprofessors are expected to apply for professorships at other universities during the latter part of the six-year period, as their universities should not offer tenure themselves (unlike in the tenure track schemes used, e.g., in the USA). The number of academics appointed as 'junior professors' in Germany has risen from ca. 900 in 2008 to ca. 1600 in 2014.
Other positions
Honorarprofessor (Hon.-Prof.): equivalent to the Dutch Extraordinary Professor, non-salaried. An honorary title (not related to any sort of honorarium!) conferred upon the person by a university for particular merits, often earned outside university or through long-term commitments (e.g., continued teaching) at the institution that confers the title. A Hon.-Prof. is obligated to lecture on a small scale. However, this is sometimes circumvented by title holders, especially since the title became popular among executives.
außerplanmäßiger Professor (apl. Prof. or Prof.): either a tenured university lecturer or a former Privatdozent to whom the title is given if she or he has done excellent research before and after the Habilitation but has not attained a regular chair. The word außerplanmäßig (extraordinary or supernumerary) literally means "outside of the plan" and denotes that he is not paid as a professor but only as a researcher. Nonetheless as a member of the faculty he or she is obligated to lecture and conduct examinations and often supervises doctoral theses. This position is common in particular in medicine but also in social and cultural disciplines.
Privatdozent (Priv.-Doz. or PD): member of a faculty who has passed the Habilitation; this title may also be awarded to a former Juniorprofessor and is comparable to the English-American assistant or adjunct professor. A Privatdozent is obligated to lecture and conduct examinations (often without pay) in order to keep the title and is allowed to supervise doctoral theses.
Lehrbeauftragter a paid part-time (for example 2 hrs per week in a semester) teaching position for scientists in general with non university position who often hold a doctorate; Lehrbeauftragter is sometimes comparable with an adjunct assistant professor or an adjunct associate professor (US). It is not considered a professor position in Germany.
Vertretungsprofessor: is an interim professor who officially represents a vacant chair for a limited amount of time, mostly 1 or 2 semesters. Very often a completed Habilitation is required. It is comparable with visiting associate professor (US). Some academics use this job as a changeover position before getting this particular job in a tenured way or before getting a tenured professorship at another institution.
Gastprofessoren: A visiting scholar. If he or she lectures he or she is sometimes also called a visiting professor. Further, a visiting scholar can also work as a Vertretungsprofessor.
Seniorprofessor (distinguished senior professorship): A special arrangement where a professor close to retirement is freed from the requirement to lecture and does only research. His or her salary is already paid from the pension fund, as if he or she retired early, and part of his previous regular salary is often used to hire a young successor to gradually take over the Seniorprofessor's work.
Other professors
Employment title outside the universities: In Germany, some civil servants like directors of certain public museums or research-oriented public institutions bear an employment title which contains the word 'Professor'. Examples: "Präsident und Professor des Bundesinstituts für Risikobewertung" ("President and Professor of the Federal Institute for Risk Assessment"), "Präsident und Professor der Stiftung Deutsches Historisches Museum" ("President and Professor of the Foundation German Historic Museum").
Gymnasialprofessor (Professor at Gymnasium): In some German states, senior teachers at Gymnasium, which is a type of secondary school, were also designated Professor in the late 19th and early 20th century. In Austria, tenured teachers at Gymnasium are still called Professor.