There are basically four different types of DJ. Obviously there are sub-categories of each type. But, for simplicity sake let's just look at the four major categories for now. Each of these types of DJ has it's place in music, but only one type is really suited to perform at a wedding. So, let's take a look now at the four types.
The first type is a producer / remixer. While this person is typically not even a DJ, they get tagged with the title, especially when they choose to perform live. This person specializes in creating new music, typically EDM or Electronic Dance Music or remixing other artists music to make it more suitable for other DJ's to play. David Guetta, Skrillex and Deadmau5 are some examples of this type of DJ. This person is obviously not what you want for the typical wedding reception and they are probably not interested in doing weddings anyway.
The second type is a club. This is a DJ who specializes in mixing, beatmatching and possibly even scratching. They will play at dance clubs and usually only play one genre of music on any given night. That may be techno, drum and bass or dubstep. Again, this type of DJ usually is not what you would want at your wedding. The "majority" of your guests are not going to be into this scene. The club DJ will sometimes moonlight as wedding DJ's though to make extra money. Just be careful in hiring one of these, since they typically do not do as well in the wedding environment.
The third type is the Mobile DJ. This is definitely getting closer to the mark. This DJ will typically have their own sound system and lights, just like a Wedding DJ. They usually play in smaller bars or lounges. They will sometimes play at parties and other types of events. A mobile DJ will most likely have experience as an MC also. But again, they are probably more comfortable playing a single or limited amount of genres.
The fourth type is of course, a Wedding DJ. This is a mobile DJ that specializes in weddings. They are adept at not only playing all genres of music, but can also emcee your event. A good wedding DJ will also assist you in planning your event, at least the entertainment portion. This type of DJ may do other types of events such as school dances or class reunions, but their specialty is weddings. This is their bread and butter and will fill most of their schedule.
Since your DJ will be a large portion of your budget, be sure to choose wisely.