Hi,

Suppose I enter Thailand and get a tourist stamp on my passport. Can I leave Thailand a few days later and re-enter before the end date of my tourist visa. I'm planning a small trip in between.

If you mean you arrive on a visa exemption (not a visa from a consulate) you can leave and re-enter no problem, you'll get a new 30 day stamp when you re-enter.

If you have a visa from a consulate it will be cancelled when you exit unless you obtain a re-entry permit from immigration which will keep your permission to stay alive.

You will get 30 days when you re-enter by air without a visa or 15 days when you re-enter at a land border without a visa. If you have a real tourist visa from a consulate you can either come back on a visa exempt entry (if your nationality qualifies for that) or get a re-entry permit.