Basically intellectual property laws state that ideas or creativity are a marketable good, and that the person who actually creates something has ownership of the creative output, and is able to sell or license their own creativity as a marketable property, and are entitled to compensation. Problems come in when people who do not have ownership or assignment of rights to the creative output find ways to profit from the creativity or ideas of others. This comes in the form of bootlegged music and movies, written expression, sharing of files with people you don’t know, etc., or any act that deprives the lawful owner of the intellectual rights.
Some countries have zero recognition of intellectual property rights and tolerate the illegal replication and sale of material. What is the most problematic about ACTA seems to be the proposed agreement with ISPs to remove illegal content from individual’s computers. Basically, it’s becoming permission for the government to search your computer without asking or a warrant.