Definition of NLHE
NLHE is short for No Limit Hold'Em. NLHE is often referred to as the "Cadillac of poker variants". There is a distinction between No Limit Hold'em, Pot Limit Hold'em and Limit Hold'em. Hold'em is one of the many variations that make up poker (Stud, Omaha, Hold'em, 2-7 Triple Draw, Courchevelle, etc). NLHE, on the other hand, is the most popular and widespread form of poker. Every year in Las Vegas there is a poker tournament called the "World Series of Poker Main Event" and it is this $10,000 US no-limit hold'em tournament that determines the world poker champion.
Other forms of poker
Often, poker variants will be written as NLHE for short. Pot Limit Omaha for example will be identified as PLO. Omaha Hi-Lo will often be identified as O8 or O H/L. Be sure to select the right options when playing online poker as your poker room may be using these shortcuts. Many people have wondered why they couldn't go all in preflop because they chose an LHE table instead of an NLHE table!