I read in a revision guide that plasmids can be put into liposomes to carry genes into human cells for gene therapy. How can this work? Can human cells express genes located on plasmids? I thought they would have to integrate themselves into the human chromosomes in some way, and that only the use of viruses as vectors would lead to such integration?

thanks the phrase you are looking for is episomal expression. Plasmids will replicate with high levels of expression in mammalian cells without the need for integration into the host genome. The downside of that is that the vector and thus the inserted gene will not normally be expressed in daughter cells.

David is correct.

This technique is used a lot to try and study (and work out) what different proteins do in the body (although not on humans).

You can also artifically silence gene expression using siRNA and shRNA.