In American Football I sometimes hear the defensive tackle getting called the "nose tackle". I know that it has something to do with the 4-3 3-4 schemes but can somebody give me an explanation along with a reference or better yet a few current players that are considered "nose tackles"
In a 4-3 defense, there are two defensive tackles (DT) in the interior of the line, while in a 3-4 defense, there is only one tackle - the nose tackle.
Usually, the primary task of the nose tackle is to occupy more than one blocker in the interior of the line to allow linebackers to make plays. In contrast, the role of 4-3 tackles depends heavily on the design of the defense - some 4-3 DT's are pass rushers (example: Warren Sapp) while others also occupy blockers in more of a run-defense role.
Vincent Wilfork is listed as an example of a 3-4 Nose Tackles in this Pat Kirwan article on nfl.com. In the same article, Pat Williams is listed as an example of a 4-3 nose tackle. The article describes the roles of these two types of nose tackles:
A 3-4 nose tackle is asked to command double teams and prevent guards and centers from getting to second-tier blocking at the linebacker level. A 4-3 nose tackle is required to penetrate the inside "A" gap and often loop around and be responsible for containing the "C" gap. Then there are 3-4 defensive ends who really aren't ends, but instead work down into the "B" gap to defending power running plays. Then there is the hard-to-find, 4-3 three-technique tackle who is asked to penetrate and be disruptive.
This guide to defensive schemes provides good background material on this topic, including a list of roles of various defensive positions for each scheme, while this 2011 NY Times article contains a list of 10 top defensive tackles.
On the offensive side, the seven "linemen" are the ends, tackles, guards, and center.
The defensive line typically consists of two "ends" (who face the offensive tackles), and two "tackles" (who face the offensive guards). The reason this arrangement works is because the defense has three "linebackers" behind them, who can help stop either the run or the pass.
Teams that are primarily worried about the passing game may elect to create a fourth linebacker--by using a three man line. The men at the end are still called ends. The man in the middle is usually called a nose guard. He is typically a very big man who has to do almost the job of two tackles in containing the run, so that others can concentrate on an anti-pass defense.