I see "tax at source" used a lot. Can someone define?

Does it just mean having a salaried job?

no its tax taken out of someones salary before it is paid to them.

most commonly for g permit people.

Sorry, that's what I meant.

A salaried person is in a "tax at source" situation. And presumably a self-employed person is a "tax declarer"? Is that right?

Not right.

B-permit holders pay tax at source.

If you are a salaried employee, with a C-permit or a Swiss passport, then tax is not deducted from your monthly pay. If you have a B-permit, then it is. But if you don't have tax deducted at source (i.e. by your employer), then it's up to you to make sure you've enough in your savings to pay the demand when it arrives.

I'm a B-permit holder with an Einzelfirma. I am NOT taxed at source, correct?

do you pay your tax yourself or is it removed before you're paid (i.e. AT SOURCE)?

I had an Einzelfirma only for 2009 so far, so no taxes paid at all!