It's rather vague, but here's what the NFL Rule Book says in regards to the San Diego Chargers situation with sticky towels.
Adhesive, Slippery Substances
(h) Adhesive or slippery substances [are illegal] on the body, equipment, or uniform of any player; provided, however, that players may wear gloves with a tackiﬁed surface if such tacky substance does not adhere to the football or otherwise cause handling problems for players.
(b) For violation of this Section 4 discovered while player is in game, player will be advised to make appropriate correction at the next change of possession; if violation is not corrected, player will not be permitted to enter the game. Provided, however, if the violation involves the competitive aspects of the game (e.g., illegal kicking toe of shoe, an adhesive or slippery substance), player will be suspended immediately (removed from the game for one play) upon discovery.
(e) Towels, provided they are white licensed towels approved by the League office for use on the playing field. Players are prohibited from adding to these towels personal messages, logos, names, symbols, or illustrations. Such towels also must be attached to or tucked into the front waist of the pants, and must be no longer than 6 by 8 inches (slightly larger size may be issued to quarterbacks, or may be folded to these limits for wearing in games). A player may wear no more than one towel. Players are prohibited from discarding on the playing field any loose towels or other materials used for wiping hands and the football. Streamers or ribbons, regardless of length, hanging from any part of the uniform, including the helmet, are prohibited.