If you are thinking about which guitar is best for you, the two major brands are Fender and Gibson. Most other cheaper alternatives are usually based on one of these brands.
There are lots of differences between the models but the main difference from a playability standpoint is that Fender necks are a bit longer and therefore tend to suit slightly bigger hands. If like me, you have quite small hands, the slightly smaller Gibson necks are usually more comfortable to play. If possible, try to play a guitar before you buy it as every person is different and a guitar is a very personal choice.
For younger children 3/4 size guitars are available but a good age to start thinking about a full size guitar is around 10 or 11.