exercising breaks down tiny little fibers in your muscles, when they rebuild themselves your muscles get bigger but the tearing down can burn or create soreness.

Along with mekare's answer, it is a build up of lactic acid.

Two things. Lactic acid build up, and your muscles actually stretching.

when you excerzise alot it builds new muscle but first it has to remove the muscle you already have to make a bigger one

Because your not fit

the muscle fibres break down which causes a burning sensation, i think it can also be a build up of lactic acid.

Some of the previous answers are good and correct, nut a slightly more detailed explanation is given here. When you exercise anaerobically, such as weightlifting, or sprinting, your muscles are burning a supply of glycogen to provide the chemical energy needed to allow them to contract. They burn this in a metabolic process without the use of oxygen - hence ANanerobic (without air). This results in the production of lactic acid, which is toxic to the body and produces the burning sensation. After you stop contracting the muscle, the belly of the muscle relaxes and the pressure on the blood vessels is reduced, allowing increased blood flow which results in the supply of new oxygen and the removal of the lactic acid buildup, so the burning sensation diminishes. The next day however, you may ache when the muscle is stretched (often in areas where you didn't think you were using the muscle - after a half marathon my abs ache because of the constant role they play in maintaining balance and assiting me in breathing). This is not due to lactic acid, this is due to microtrauma to the muscle fibres. Your muscles are made of lots of little fibres all runing alongside each other - when the muscle contracts, the fibres shorten in length and so the mass of the muscle is contained in a shorter distance, hence why the muscle appears to get thicker in the middle. This microtrauma happens constantly, day in, day out. However, normally you don't notice it becaue it is on such a small scale - similar to you not noticing that you burst thousands of small blood vessels every day. However, after a period of severe exercise, the microtrauma is on a much larger scale - a large proportion of the fibres have been damaged and you feel an ache. The muscle fibres are repaired (assuming enough protein in them diet) and the muscle may hypertrophy if subjected to enough stress over time, that it, it grows to accomodate the larger load you make it work against, hence why lifting heavy weights causes your muscles to increase in size.