How to Use Kayak Rudders and Skegs
Believe it or not, the main purpose of rudders or skegs isn't to turn a kayak, it's to keep a kayak running straight when you're paddling with a crosswind.
A kayak will naturally want to turn into the wind, something called weathercocking. A rudder or skeg is used to fight your kayak's desire to do so. Without them, when you're paddling in wind you can find yourself taking much harder strokes on one side to keep your kayak running straight.
Rudders are way more popular than skegs because they can also be used to help steer a kayak. Rudders flip down from their stored position on top of the deck by use of hull lines that are found alongside the cockpit. They're then controlled by your foot pedals.
Skegs are stored in a skeg box that is embedded in the stern of the kayak. They're deployed by use of a slider found alongside the cockpit. Because skegs don't swivel from side to side, their control comes from the depth at which they are set. The more your kayak wants to weathercock, the deeper you'll set the skeg. Since skegs are really only useful for tracking over long distances, you generally don't find them on recreational kayaks, only some touring or sea kayaks.
If you expect that you'll be paddling in windy conditions, you'll want to consider getting a rudder. Many kayaks now come with a rudder mount in the stern even though there may not be a rudder. If there is a rudder mount, you can have a rudder installed anytime that you decided that you need one.