pgdump - What happens to data modifications (INSERT, UPDATE, DELETE) while pgdump is running?

If I run pgdump, and while it is running, execute INSERT, UPDATE, DELETE
commands, what will happen to the pgdump results?

- Is pgdump all contained in a single transaction, so it just gets the
'before' data state,
- Do some of the modifications end up in the dump, but not others depending
upon where the
  dump is at that exact moment?

The pgdump documentation is not clear on these transaction concurrency
issues.

Thanks, Jack