Triple sec is a liqueur made from dried orange peels. You'll probably use it most for margaritas or tiki drinks that call for dry curaçao (which is basically an old-school name for triple sec) or side cars.
In my opinion, triple sec is one of those bottles you don't want to go too cheap on. You can find $6 triple sec and most liquor stores but it's mostly sugar water. If you buy really ice tequila and crappy triple sec to make margaritas, then your margaritas are going to be crappy.
Cointreau is truly "the standard" for triple sec and it is tasty stuff. It has a balanced, dry sweetness vs. "orange cream-sickle" like the cheap versions.
However, I've also been pretty impressed with Patrón's Citrónge Liqueur. It usually less expensive than Cointreau and still on the "dry" side when it comes to sweetness. I tend to use Cointreau when I have just a few friends over, and I'll go with Patrón Citrónge for big parties.