The Anti-Counterfeiting Trade Agreement (ACTA) is an international treaty currently being negotiated by the USA, the EU, Japan, Mexico and some other countries. The name is rather bland, deliberately so, and the treaty has been negotiated in secret because the interests behind it want to prevent criticism.
The people behind ACTA are the content distribution industry — the MPAA, RIAA, IFPI, etc — and their goal is to prevent the internet from being used to make unauthorised copies of copyright material. Needless to say, this is impossible without crippling the net.
Here’s a video that shows how ACTA will work: