How are locks created in a table?

Hi there,

I have quite an interesting query as to know how locks to tables are being created. Is this being created automatically by default in oracle, cause we have not specified in any part of our software modes to lock the table. However, the creation of locks is quite a good policy to follow, when it comes to the point of multiple user access of data.
Could any of u guide me as to how these things happen. Is it because of the query execute statements we write in VB incorporating oracle say for instance:
acrs.open"select * from emp",con,adOpenForwardOnly,adLockOptimistic

Is it due to this way of mentioning the adlockoptimistic/pessimistic that locks do get created. But this we have been noticing only since of late in our software.
Note : Our Software works on MS Visual Basic and Oracle 9i.

Kindly give me your valuable advise.

cheers
Abigail