Please avoid multi-part questions. It makes the answers harder to follow and takes us longer write them. Limit number of points you ask to 3 at most.

1. To take a number of inputs and return a value based on these.

2. You can call functions from SQL. You can't do this with procedures.

If you want to return multiple variables, this is best done with out parameters in a procedure.

You can create indexes on the result of functions, but not procedures. There is also the result cache available for functions.

3. Yes. But not if you call it from SQL (ignoring autonomous transactions).

4. It does. But this defeats the point of functions. If you're doing this, then you should use a procedure.

5. I don't understand what you mean by "real time". You're always calling functions in "real time".

6. See 2.

7. As many as you want. It's not really a good idea though. It makes the logic harder to follow.

8. You can return arrays, records and arrays of records. If you mean multiple different variables, see 4.

If you want more details, I suggest you read the documentation on PL/SQL subprograms:

http://docs.oracle.com/database/121/LNPLS/subprograms.htm#LNPLS008