As in all lowball games, pairs and trips are bad: that is, any hand with no pair defeats any hand with a pair; one pair hands defeat two pair or trips, etc. No-pair hands are compared starting with the highest ranking card, just as in high poker, except that the high hand loses. In deuce-to-seven low, straightss and flusheses count for high (and are therefore bad). Aces are always high (and therefore bad).
For example, the hand 8-5-4-3-2 defeats 9-7-6-4-3, because eight-high is lower than nine-high. The hand 7-6-5-4-2 defeats both, because seven-high is lower still. The hand 7-6-5-4-3 would lose, because it is a straight. Aces are high, so Q-8-5-4-3 defeats A-8-5-4-3. In the rare event that hands with pairs tie, kickers are used just as in high poker (but reversed): 3-3-6-4-2 defeats 3-3-6-5-2.
A special rule is that a wheel is not considered a straight: A-5-4-3-2 is simply ace-high no pair (it would therefore lose to any king-high, but would defeat A-6-4-3-2).
It's called deuce-to-seven low because the best possible hand is 7-5-4-3-2, followed by 7-6-4-3-2, 7-6-5-3-2, 7-6-5-4-2, 8-5-4-3-2, 8-6-4-3-2, etc.
When speaking, low hands are referred to by their highest ranking card or cards. Any nine-high hand can be called "a nine", and is defeated by any "eight". Two cards are frequently used: the hand 8-6-5-4-2 can be called "an eight-six" and will defeat "an eight-seven" such as 8-7-5-4-2.
wild cardss are rarely used in deuce-to-seven games, but if used they play as whatever rank would make the lowest hand. Thus, in 7-6-Joker-3-2, the joker plays as a 4, while in Joker-5-4-3-2 it would play as a 7 (a six would make a straight, and an ace would make ace-five high).