I don't fully understand the question. I think you could be confused about what a DJ does. Also, mixing songs as a DJ is a form of art, so basically anything can be done, as long as it sounds good to you. But you seem like you want an explanation of traditional, smooth song crossover mixing.
So firstly, to make this answer a full one I'll try to explain it simply (if you know all this, skip to 'secondly').
Mixing two songs (from one to another, smoothly with no lull) in it's most basic form firstly involves the two songs. Each song can be any style of music, from The Who to Tiesto.
There are three basic things we need from the two songs:
- Closely Related BPM: We need them to have a closely matching, consistent tempo or half tempo (measured in BPM).
- In Harmony: As they will be playing simultaneously, We need them to sound good together harmonically. This can be tested by ear or by finding the key each song is in. (see circle of fifths for key matching)(! not essential !).
- Time Signature: We need the amount of beats in a bar to match(occasionally, this can be somewhat overcome if you are clever).
So, you have your two songs matching the criteria above. While one song is playing, you have to change the tempo of the second song so that it matches the first song. You do this with the speed control slider on the turntable/CD player. You also make sure the level of the second song matches the first. You do these adjustments discretely, by listening on headphones via a cue channel specifically designed for you to match the songs before you output the second channel to the main output.
The Final Countdown
When you have the tempo matched(within reason, slight adjustments can be applied on-the-fly), the next thing to do is to find a cue point. This will be the point at which you will start the second song when it is time to start mixing the songs live. For example, the first kick drum hit(the downbeat in the first bar). It's your preference when you start the second song playing from this cue point, but it should be at the same point in the bar progression as your cue point is. But basically, you should start it when you feel it should start. It's a good thing at this point if you have played these two songs before, so you have an idea of where to cue and when to start the second song.
Controlling The Crossover
So, you have started the second song. You have quickly monitored the mix discretely in the headphones to check all is smooth and aligned. If necessary, you can cut some of the bass from the second song(two full energy bass lines in the same range with kick drums can clash disastrously). At this point, you are ready to start bringing in the second song either by easing up the fader for the second song's channel, or by using the cross-fader. The amount of time you take fading in and holding the two songs in the mix together is up to you and the arrangement of the songs. Slight adjustments may need to be made at this point; level, tempo, EQ etc., and the second song becomes dominant. At some point, the first song will need to be removed from the main output. Depending on the situation, this can be done smoothly or quickly. You can fade it out with EQ, effects or simply with the fader or crossfader. Either way, as a rule the removal of the first song should not leave a noticeable lull or dip in perceived energy. This completes the crossover process. The second song should now be playing on it's own with correct level and EQ.
This whole process has begun and ended in a few seconds in some of my sets and in others, it's lasted minutes.
Secondly, Your Questions
DJing is an art form. There are many different styles of DJ, from basic to advanced, from analog to digital. It's hard to say which type you are speaking of. On my mixer, I have a sampling channel, so you can indeed store short loops that can be played back at any time. But I'm not really sure that this is what you mean by "background music".
As for the big difference in tempo, this is occasionally done, and there are a few options:-
- If one song is half the tempo of the other(or close to), then they can be mixed creatively. This is often done with Hip-Hop and Drum 'n' Bass, with fantastic results.
- If there is an intro in the second song, the first can be removed from the mix creatively, maybe even with effects. This can also have a good outcome if done right.
- With new pitch locking technology, it is now possible to speed up a song quite dramatically, and still have a playable song. So you could simply speed up the slower track and although I don't like to slow a song down, it is possible to 'meet in the middle'. With some creativity, the song's original tempo can be restored after the mix. But the previous option is usually favored with big differences in tempo.
I hope this helps.