It's important to feel safe and secure when it comes to an exterior/entry door. I don't think anyone would feel comfortable installing a regular door knob with no type of lock on an exterior door, I guess unless you live out in the middle of nowhere and I am guessing that probably means you leave your keys in your car too. Anyway, not everyone lives where there are no neighbors in sight. So it's important to find a lock(s) that makes you feel comfortable when leaving the house for the day or maybe days. Believe it or not there are a lot of door lock options for an entry/exterior door, not just your basic door knob that has a key.
The most common combination is probably a basic single cylinder deadbolt with a passage function knob or lever. A passage function means it has not type of lock on the door knob or lever, the deadbolt is how you would keep your door locked. You could choose a double cylinder deadbolt if you wanted, but that mean's it will be keyed on the outside as well as the inside. It's not recommended for most doors, as it could be a fire hazard.
Sideplate locks are a little fancier option with so many different plate options and you get to pair it with your choice of door knob or lever and choose whatever of the many finishes. These work basically like the first option (deadbolt and passage function knob or lever) except it has a fancy plate in between. Sideplates are from Emtek. The picture above shoes a sideplate with a CC of 3 5/8" (which is not standard prep). 5 1/2" CC sideplates are available on the right hand side of this page.
And last but not least, you can also find many many styles of Handlesets. These are pretty popular as well for a front door, maybe not so much for a back door or side door, but the front door it'll sure make a statement. Most handlesets will only have the above deadbolt to keep the door locked, but if you are wanting added security and have 2 locks you can check out the two point handleset locks.
Don't forget to check out the keyless locks, which could take place of several of these options.