I'll try to provide some things the previous reply hasn't, while skipping what they did mention:
Is it (always) compiled?
Well, the interface for the programmer says "Activate", not compile, but that's because there is built in version management. When you Activate something, it compiles it, and makes the source and binary versions the "active" version. If someone tries to run source that isn't compiled, it will automatically be compiled as they run it.
Typestem : Is it strongly typed?
Basically, yes. ABAP is like a strange cross between Pascal and COBOL. There are "Field Symbols", which are like pointers. There are also now generic types. (And generic field pointers). One of the coolest things is that there is no difference between ABAP types and Database types. Any table you make will become a structure type automatically. That is to say, ABAP is integrated with the database in a way that almost no other languages are. You can also write SQL, which is actually part of the ABAP, as opposed to being just a string, like in other languages.
Inheritance : single / muliple, interface-like structures ?
There are interface structures, I don't usually use them. If you made a lot of re-usable classes, they would be a good idea, though.
Collections : Has it collections apart from arrays ? Has it generic collections? Does it use List comprehension ?
Better, it has "Internal tables", which are basically a type of dynamic array. They can be declared from almost any type, including those in the data dictionary. They can be looped, sorted, etc. There are several types, including hashed and sorted variants.
Any Exception handling?
Both OOP and non-OOP types.
Anything remarkable as oposed to other well known languages?
As the other poster said, a lot. It is very good at anything to do with database operations, and re-using complex structures and data types. It is naturally is cross-database and cross-platform (OS, and processor). It has a very good version management and transport system. The whole SAP system has very good multi-language support. You can get simple screens with appropriate titles and selection boxes automatically, which means that you can do less programming and more work. You don't have to map between DB types and language types, etc.
Things it's less than good at:
1. Numbers are typically stored as ASCII, this makes it less than fast at math in many applications.
2. Most of the data structures are very normalized, meaning your data may be spread across 50 tables sometimes. "Star" queries are very common. Built in functions to retrieve data (f.e. Logical Databases) are your friend in those cases.
3. SAP tried to be everything to everyone, so there are a lot of configuration options, etc., and sometimes a function doesn't do what you expected it to based on past behavior.
4. ABAP is sometimes very verbose. Try using the "pattern" button on something simple like GUI_DOWNLOAD.
5.SAP was very ambitious with what they took on, and thus ran into limitations of hardware, operating systems, and RDBM systems early-on. Thus, they have legacy kludges to deal with it that aren't pretty. (Pool Tables, Cluster tables, etc.)
6. When you activate a program, it doesn't do full compatibility checking with the function modules used. There are situations where something will activate with no problem, but then crash at run-time, even in cases where it could have been caught at compile time.