Question about the Internals

I'm a consultant for a large company and they are looking at a database to
use.

I made the suggestion about going with Postgresql.

I told him about it's MVCC design, he liked that ideal, but wanted to know
exactly HOW did it handle the multiple versions of records.  He's concerned
because he was burn very early on by another database that was MVCC (won't
mention any names) and his system became corrupt.  That database has since
improved and fixed all of their problems.

But the question is, how does Postgresql handle the multiple versions?
How does it handle the records BEFORE they are committed and how does it
handle the records AFTER they are committed that allows different users to
possible have a different view of the data and indexes. And also how does
the transactional part of the system place a role?

Any help here would be great appreciated!

Thanks