Most smart phones these days a accelerometer in them, some even have both a accelerometer and a three axis gyro in them. Can anyone tell me the difference between the them?

Me and another person are currently planning something that requires the phone to be able to sense if its lying flat or on a angle and if its on a angle what is the size of that angle.
If we were to use a phone with just a accelerometer would it be capable of doing this or is a three axis gyro required for this?