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.