Some dating sites, such as eharmony, only offer compatibility matching and therefore do not allow you to communicate with others at all unless you were introduced by the sites' matchmaking systems.
Services such as these may be ideal for you if you prefer to remain anonymous.
If you prefer lighthearted fun and flirtation you may also enjoy sites such as Zoosk that cater to singles that prefer to keep things casual.
And if you just want to meet someone and see where things go, using the dating sites with the largest possible user base, such as Match.com, will provide you with the best chances of success.
While you can do without most of the extra features they offer, some dating sites have features that really stand out and should be taken into consideration when you decide on a service.