According to Wiki, GMO's are "any organism whose genetic material has been altered using genetic engineering techniques."
Humans have been responsible for modifying the genes or expression of genes of organisms for millennia through a different method - artificial selection - where certain desirable traits are selected for breeding future generations. This process has given rise to the variety of vegetables and domestic animals we have today.
It has some problems, for instance, some dogs cannot give birth naturally and rely on cesarean births. Others are prone to certain diseases.
It's my view that whether genetic modification is achieved via selection or via insertion of genes, we have a responsibility not to produce abominations.
In terms of food, fruits are seemin to be bred with more and more sugar, less fiber and possibly less nutrition. Not sure if this is consumer driven or if plants breeders have a preconceived idea that fruit should be as sweet as possible.
Lots of GM is focused on increasing tolerance to pests or resistance to herbicides which makes things easier for farmers.
Based on various studies, no ill effects have been reported from consumption of GM foods but it remains one of those things that makes people feel icky.
Humans are a powerful influence on the Earth and it's biomes. Most of the domestic plants and animals we come into contact with have been shaped over centuries or millennia by the hand of humanity and we can be grateful for that as we don't have to eat carrots that resemble a piece of wood or tomatoes that are poisonous. GM is the next phase of humanities influence over the way domestic plants and animals are and I trust that most of the time, advancements made in this regard will be useful/benign.