How does a tax deductible slip from Goodwill work come tax time?

Certainly it's acceptable documentation....for those types of donations you need to document. I keep a copy of the receipt with my return--the information goes on the tax form that deals with charitable contributions and the type of contribution.

It does not need to be attached to a return or anything. Just, if you are audited, you have to produce documentation for the amounts that you claimed as a deduction based on contribution. Some contributions, basically those above a few hundred dollars, require specific types of documentation or proof.

Certain types of donations require special types of paperwork...cars, stocks, etc.

See the link for complete explanations, limits and other requirements for having a supportable charitable contribution deduction.