In every department the top is the Permanent Secretary. This is the head civil servant in a department and will have his/her own private office like Ministers do.
Then there will be a few Director Generals. These will head up 'groups' of different directorates and have responsibility for the general strategy of a vast policy area.
Each directorate is headed by a Director. This job brings a lot of responsibility and a lot of pressure as you are highly accountable, Directors work long hours and have pressure from Ministers. Once you are at this level you are probably sacrificing salary to stay in the civil service as like for like jobs in the private sector would pay more. A director would probably earn £80-90k but this would be a similar level job to being a partner in a large firm in the private sector, many of whom would earn much more in their total compensation package.
Then within each directorate there are deputy directorates headed by Deputy Directors. This is often called Grade 5 and marks the start of the Senior Civil Service. If you get to Grade 5 you have done well and had a very successful career, and will make important decisions. A Grade 5 will earn about £65-75k a year.
To get above 5 you need to be really special and have a lot of extensive and diverse experience.
Below the Senior Civil Service are Grade 6 and Grade 7. Grade 6 is about £50-60k a year and Grade 7 is about £45-50k a year. This is where most of the graduates on the Fast Stream will end up spending their careers.
After this you go from numbers to letters. The order will be
SEO (Senior executive officer)
HEO (Higher executive officer) - around £25-32k. If you enter in the Fast Stream you come in here.
The vast majority of civil servants are in these grades:
EO (Executive officer) - around £24-26k
AO (Administrative officer) - around £18-22k
AA (Administrative assistant) - around £15-17k
Fast Streamers are supposed to accelerate from HEO to Grade 7 quickly (3/4 years), whereas the climb to Grade 7 outside the fast stream is a long and weary one.