The gap between the guard and tackle is called the B gap. If you are a defensive tackle lined up in the B gap, but shifted over a bit towards the guard, you're called a 3-technique. If you were lined up in the same gap but shifted over a couple feet to line up on the tackle's shoulder, you would be a 4-technique. If you line up directly facing the center, you're called a nose tackle.
Nose tackles are two-gap players, and are typically very big and very strong men, usually 350 pounds or more and 6'5" or taller. These guys have the responsibility of clogging up the entire center of the field, of keeping the center and at least one guard busy, and thereby protecting their middle linebacker. The nose tackle will also be responsible in passing plays to push the center back towards the quarterback so that the quarterback cannot step up in the pocket and evade the rush of the defensive ends. A good nose tackle can be hit simultaneously by 650 pounds of center and guard and will not budge as much as one inch.
A 3-technique tackle lines up between the offensive guard and tackle. A 3-technique tackle is supposed to run through his gap immediately. He is a 1-gap player. His job is not to block or get tied up in a block, but rather to be athletic and get himself into the offensive backfield and disrupt their plans. Because of this a 3-technique tackle is a lighter more athletic guy than a nose tackle, typically weighing more like 290 to 300 pounds.