I cannot speak for all DJs, because there are so many ways to do it nowadays, but here is how I am DJing:
First things first, I am DJing in a very classic sense, in that I only use turntables, a DJ mixer and vinyl records. soemtimes I throw in a little bit of effects, but very selectively.
Before I head to a gig, I spend an afternoon preparing the selection for the night. I listen to music, evaluate the mood I am in today, practice a little bit and think about my expectations about the party (location, sound, crowd...). With this in mind, I select 50 - 80 records I put into my recordbag, depending on how long I can/have to play.
I try to always take a diverse selction with me, so I can react more to the setting. I also try to bring lots of different energies to change the mood if needed (or wished for). So in my record bag, you'll usually find a selection of house (mostly chicago related), (dub) techno, electronica/idm and sometimes even new wave/80s. I try to put music into context so I'll blend tracks from the past 30 years. recently, I've been buying lots of tracks from the 90s and early 00s. But this follows no strict pattern, if music is good, I'll play it, no matter if it's from 1993 or 2013.
Arriving at the location, I try to sense the ambience, the mood of the people (this is all very subconcious, and not analytical) and when it is my turn to play, I usually try to combine a smooth transition from the last set while still brining in a new start. I often slow down things at first, because it makes it easier to build your own set and influence the vibe of the party, and then I will gradually work on bringing things to higher energies.
I think while DJing, it is most important to have something to tell... when I hear a DJ playing the same style in the same tempo and energy for two hours without any variance whatsoever, I want to leave the party, because it just gets boring. For me, DJing is a way to express my feelings by putting the music of others into context (between tracks but also between me and those who listen).
This was a view on my emotional connection on the act of DJing (something which is often overlooked, because many people view DJs just as a human jukebox rather than a legitimate artist (well many DJs actually are, so I can totally see where that comes from)). now I will talk a litlle bit about the technical side of it.
As I've said, I DJ in a very traditional way, turntables, vinyl, nothing fancy. the act of mixing is explained very easily, you basically have Turntable A & B, a mixer M and Headphones to preview the tracks you will be playing.
With the mixer, you can set the volume of each track/turntable/device individually, as well as having an equalizer for each channel. For example, I can put the turntable settings to 100% volume for Turntable A and 50% volume for turntable B, so you will hear track A more loud while track B stays in the background
With the equalizer, you can manipulate the frequency spectrum of each track. For example, if you cut off the low frequency of track A, you won't hear the Bassdrum and bassline of that track anymore (or much less loud, depending on the type of EQ).
With the crossfader (which I hardly use) you can swap between A <--> B, when the crossfader is in the middle, both tracks have the same volume. The volume for each channel can be (usually) adjusted with line faders, which are vertically aligned.
Well my general mixing procedere is along the following: I have track A playing on the PA, heard by the audience. channel B is mute. I put a new record I want to bring in onto turntable B. I put channel be into the cue channel, which is connected with the headphones I am using. So I can listen to track B without the audience hearing the new track yet. I then beatmatch track B to track A, by adjusting the tempo, and dropping the new record in time with the groove of track A.
Sometimes it is easy to beatmatch, sometimes it is a lot of work, depending on the music I am working with.
To adjust the tempo, I use the pitch control of the turntables (+/- 8%), and I will also use my fingers, punctually speed up or slow down the record, if needed.
If i feel that the tempo is adjusted well enough and that the tracks actually fit each other, I will cue again at the right moment and then start to bring track B in, by turning up the volume using the line faders. I use the equalizer to emphasize certain sounds of the next track, while hiding others. Then step by step, I bring in more of track B and, subsequently start to fade out track A, until track B is the only one audible to the audience.
Then I'll pick the next track out of my record bag and repeat.
How a mix plays out really depends on the tracks you are combining. Sometimes, I make smooth, very long (2min+) transistions, sometimes the mix works better, if it is done in 30 seconds, sometimes it is best, to just cut at the very right moment (this is the hardest I think, it is not so hard, to mix two similar, grooving techno tracks for a long time, but finding the right moment to cut is very advanced in my opinion).
Sometimes I also cut in certain elements of track B. for example, I will sometimes cut in the snare drum of the next track by using the line fader as a rhythmical tool.
Well learning the techniques of DJing and finding your style takes quite some time, but I really believe that the DJ has to have an emotional connection to the music and the will to share something with the people he is playing for and that makes all the difference between a regular and a talented DJ (in my opinion).
well this became quite a long post, but I hope that you can take something out of it. DJing is very important to me and I feel that it is not very well understood, even by people who attend parties with dance music often..
greets
