If I lock a table, and another program tries to access it, what happens? ... Does it just block until it becomes unlocked, or is it supposed to return a failure?

This very much depends on what sort of lock you take on a table. If you take a READ lock, then everything else will still be able to read the table also, but any updates to it will be stalled until you release your lock.

If you take a WRITE lock, however, then nothing can read or write the table(s) concerned until you release your lock.

All locked queries will just stall, waiting for the lock to release, rather than failing.

Tony

Thanks a lot. This answers my question and I know that I can safely WRITE lock my table for a quick operation and I don't need to provide error handling for other programs that access this table. Many thanks.

But if you are grabbing several locks be careful about what order you do it in. If programs X and Y grab locks on tables A and B respectively, and then try to lock B and A, they get into this pathetic staring contest called a deadlock. This is generally a Bad Thing...

At this point I have to admit that most of the tables were designed by me, back when I knew less.

A decent primary key on a table can save a whole load of heartache later I can promise you :)