what is olb files?

HI All,
Could you please differentiate between a tlb and olb files?
Right now I am working on an application which contains lot of olb files.Just we are refering them on to our application and doing the work.
All the libraries were developed using c++(with com)?
we used to develop the dlls which refers those librarys.
Our client environment is VBA.

Can you please suggest be with a best book which gives more insight about the interface based programming using vb?Because all the libraries we are using is having interfaces only?(Calss,coclass and abstract class).

Please help me with a sample code or document about more info on how to build a class modules using visual basic?


Thanks&Regards
chandu