hi,
i'm a beginner with UAVs and i'm reading trying to implement my first project
i'd like to know the type of sensors used to keep the UAV from not hitting anything while flying
thx 
From what I've read and seen most of the collision detection stuff that you see on youtube etc is using external sensors. In this TED Talk, Vijay Kumar mentioned that an overhead array of video cameras is what they use to feed their quads positional information about them and their surroundings http://www.ted.com/talks/vijay_kumar_robots_that_fly_and_cooperate.html and a google search shows up different wireless networks where swarms of UAVs share their positional information with a ground station to ensure no two inhabit the same space (though that's not useful for obstacles that don't share that information).
Found this link on DIYdrones (http://diydrones.ning.com/) about a study on how insects do it http://www.psicorp.com/library/SR-1450.pdf (heavy going, but interesting stuff!)
For the most part it appears the most important sensor to link into your UAV for collision avoidance is still a human
Digital elevation data (usually DTED data), baro-altimeter, GPS, and the human operator are probably the best ways to keep from unplanned UAV landings. By knowing you GPS location you know the height of the land from the DTED data and use the baro-altimeter to stay above the ground. The human operatror comes in by not telling the bird to go to low or having the situational awareness that there are tall trees in the area that you need to fly over that are not in your DTED data.
In a nut shell that is the way a lot of UAVs prevent unplanned landings.
wade