Because boxers fight in an enclosed ring and that ring is big enough to allow movement, it is possible for the boxer to increase or decrease the distance between himself and his/her opponent.
Boxing is a constant negotiation over one little piece of terrain. It is the ground directly between the two boxers who are just outside of each other's reach. The entire point of a boxing match is to cross that small distance and strike your opponent without being struck yourself.
Because of this, there are three distances in boxing that you need to be aware of and how you behave in each is important.
Zone 1 - No Man's Land
You will constantly be looking for a way to cross no man's land without getting hit yourself.
In case you don't know, the term 'No Man's Land" is from the First World War and stems from the trench warfare that was used. Basically each side had a trench that ran parallel to each other. This was their front line and they had to figure out a way to get from their trench across the kill zone into the other side's trench to complete the killing.
Boxing is not a lot different than that. You are going to try and cross "no man's land" between you and your opponent so you can hit him. How you do that is part of the magic of boxing. It is sometimes a mind game, sometimes a game of force, sometimes a game of deception, but the goal is still the same - to get within striking distance without getting hit yourself.
In The Trenches
When you're negotiating "no man's land", you should be constantly alert, up of the balls of your feet, ready to move in and strike or defend against attack. This is not the time to be lazy or conserve energy. You should be coiled like a snake, ready to strike when the opportunity presents itself either by launching your own offensive, or defending and counter attacking an offensive from your opponent. You should be well protected in your guard, constantly moving - not presenting a large or stationary target.
Zone 2 - Long Distance
If you're out of striking distance, you're at long distance. Long distance is anywhere where you are basically "safe" from being hit. You are outside of "no man's land". It also means you are in no position to launch an offensive. You're too far away to hit and too far away to be hit.
It is at this distance, that you want to conserve all the energy you can. That may mean dropping your arms, stopping any excessive bouncing or using unnecessary energy in a pendulum type movement. The goal at long distance should be to be on guard, but also to completely relax and save your energy for when it's needed.
Long distance is not all that common in boxing for obvious reasons. Both boxers are expecting one another to come towards each other, or at least one of the boxers (usually the more aggressive) will usually close the distance and continue to close the distance over and over again if it opens up. The only long distance you may see in a match may be the walk from the corner to the center of the ring. If that's the case, conserve your energy there, every little bit counts.
Zone 3 - Short Distance
At short distance you are literally within the striking range of your opponent. You have moved across no man's land and are in his trenches. You are too close to receive the full extension of his/her straight punches like the jab/or straight right. You are literally head to head and body to body working the inside. This type of work will be characterized by hooks and clinching.
If you're at short distance, you want to maintain the distance or have an exit strategy. In war terms, you either take and hold ground or you launch a raid and then get out with the least damage possible. To hold ground you have to remain at short distance and fight close. Lots of body work, very tight guard, and footwork that matches your opponent's advances and retreats are the norm.
Now that you know the distances, you just have to figure out how to get from one to the other...care to share your strategies?