Disclaimer: this is cut & pasted from a rant I was having on irc earlier this evening. I haven't checked the exact mechanism for vaccine production or gene therapy against a textbook before posting this, so it's subject to memory. The basic principles are ok, though.
Genetic engineering is defined as the use of genes from other organisms spliced into micro-organisms to produce a useful product. It is not the same thing as genetic modification or "Frankenstein food". Genetic engineering uses specifically micro-organisms, that is bacteria and/or viruses - not macroscale food-type crops.
Genetic engineering produces insulin for diabetics. We take the gene that produces insulin, cut it out of human DNA with restriction enzymes, grab a handy bacterium, cut open a circular plasmid of DNA that's loose inside the bacterium, and splice the human gene in. Let the bacterium reproduce, and it produces the desired product.
Being against genetic engineering means that you want to deny life to the people who can't take animal insulin for whatever reason, because they NEED those gene-spliced bacteria to be able to produce human insulin for them. How else can we produce human insulin? It's not something that can be donated, because a healthy person produces only as much insulin as they need. It means you are against most vaccines, as vaccines are produced using genetic engineering. ("Traditional" vaccination means giving someone cowpox so they don't get smallpox. We haven't done that in a century.) It means you are against thyroxine for the many millions of people with some degree of thyroid failure, or against human growth hormone for children with restricted growth conditions or messed-up pituitary glands. To pick a less life-threatening example, it means you're against "vegetarian" cheese. Most hard cheeses contain rennet, which is an extract from the stomach of dead calves. "Vegetarian" cheese uses non-animal rennet produced by genetic engineering.
Gene therapy is another form of genetic engineering which is being trialed to treat people with inherited disorders such as sickle cell anaemia, cystic fibrosis and haemophilia. iirc, you take the correct gene, splice it into a bacterium, then splice THAT into a virus. Viruses are clever. The way they normally attack us is by splicing THEIR DNA/RNA into ours, and forcing our cells to make many copies of the virus. That's why herpes is with you forever, because it "lives" inside your cells. Given that a virus ALREADY splices its genetic material into ours - all you need to do to make it useful is to make sure that what it splices in helps in some way. So, if you put the gene that someone with, say, haemophilia is missing into a virus and introduce this modified virus to their body, it runs rampant and inserts itself into cells. That then means a small number of their cells contain the gene for making clotting factor. Only a small number will, the rest don't - but anything is better than nothing. Repeated therapies increase the number of modified cells in the sufferer's body.
I don't agree with putting fish genes in tomatoes, either - but that's not genetic engineering.