An invention disclosure is just the actual description of the invention that an inventor writes used by a lawyer to write a patent. Its the actual description of the invention before its translated into patent speak.

The difference between one of these and a patent is best summed up by one of my favorite Terry Pratchett quotes: "It's the difference between using a feather and using a chicken."

http://en.wikipedia.org/wiki/Invention_disclosure

There is a prior user defense, see 35 USC 273. Unfortunately, in its current form it is essentially useless, as it only applies to method claims, and nearly all patents contain both product and method claims.

However, the House version of the reform bill expands the defense to cover all types of claims; thus if it survives reconciliation with the Senate bill it will likely become a viable defense.