How does a VOR work?
It sends out two VHF signals. One signal is the same phase in all directions. The other signal varies in phase through the 360 degrees of the compass. Thus if you're due north of the station you receive the two signals in phase, and you are said to be on the 000 radial. If you're due south of the station, you receive them 180 degrees out of phase, and you are said to be on the 180 radial.
What do you use a VOR for?
Finding your position, following an airway (designated path between two of them), holding (waiting in place without interfering with other traffic) or flying an approach (descending towards a runway you can't see yet).
How do you use a VOR?
You set the VOR receiver in your aircraft to the frequency of the VOR station you are interested in, and then to a particular radial. An indicator on the instrument shows you which side of that radial you are on, and whether you would be going TO or FROM the station if you flew in the direction of that radial. (There are different sorts of VOR receivers, so I'm being slightly vague with regards to the "indicator".)
For example, lets say you select the 090 radial and the indicator points to the left and a FROM flag shows. That means you are southeast of the VOR. (If you flew due east you would be going away from the station, and you'd have to steer to the left to be on the 090 radial).
To find what radial you are on, turn the bearing selector slowly until the indicator centres -- shows neither a left nor right deflection -- with a FROM flag. Whatever bearing is selected is the radial you are crossing at that moment.
If you select a particular radial and then steer so as to keep the indicator in the centre, you are flying along that radial, from or to the station.
VOR vs. NDB
For a given position of your aircraft, the indications on your VOR will be the same regardless of which direction your aircraft is pointing. The needle on your ADF (NDB receiver) simply points to the NDB station, so the needle will swing around as you turn the aircraft around. That's the main difference from a pilot's point of view.
Tom775257 hasn't met this yet, but you *can* track towards or away from an NDB on a specified bearing. It just takes a bit of math, or an RMI (more expensive NDB receiver) instead of an ADF.
Also, an ADF can be used to pick up AM radio stations, while a VOR receiver can only receive entertainment radio if there happens to be a strong local station at the extreme top end of the commercial FM band.
A localizer is the same technology as the VOR, only it sends out a beam in a narrow range of directions from the threshold of a runway. Aircraft intercept the beam, and then using the same left-right indicator, follow it towards the runway. I don't think a localizer would be much use in glider navigation.