Hand Strength and The Nuts
Playing your cards is often referred to as honest play, being the opposite of deceptive play. In general you bet and raise when you think you have the best hand, call when you think you have a good chance of becoming the best hand, and fold when you think you will not end up with the best hand. Knowing which of these categories your current hand is in is called "determining where you stand" and is a fundamental poker skill.
Before the flop it is relatively easy to know how strong your cards are. The best possible hand is a pair of Aces. In general having two high cards (both Jacks or better) is a strong with the cards being suited (ie of the same suit) even better.
After the flop the first step is looking at the board and determining what the best possible hand is. This hands is commonly called "the nuts". In those happy cases that you have the nuts you can know for certain that you have the best hand.
Determining the Nuts
Determining the nuts can be difficult for first time players but with a little practice will become second nature. In general there are only four cases to consider:
a) The board has 3 or more cards of the same suit that could also make a straight. In this case the best possible hand is a straight flush. However since straight flushes are exceedingly rare it's less important to be able to spot this type of nuts instantly.
If it is not case A then:
b) The board has a pair, two pair, or three of a kind on it. In this case the best possible hand is four of a kind. If you hold one of the cards for the pair then four of a kind is impossible (although the other players don't know this) and then the best possible hand would be a full house with the highest card on the board being the three-of-a-kind component.
If it is not case A or B then:
c) The board has 3 or more cards of the same suit on it. In this case the best possible hand is a flush.
If it is not case A, B or C then:
d) The nuts is a straight if that is possible, or three of a kind with the highest card if not. Seeing if there are possible straights is more difficult than seeing possible flushes. You have to remember that other players' hole cards could fill in the gaps between the board cards. So for example if the board has 5, 7, and 9, then someone could have a 6 and 8 in the hole to give them a straight. One trick is that if the board is unpaired and contains no cards higher than a Jack, then there is always a possible straight.
There are various examples of the nuts in help.
Estimating Hand Strength
The different cases list above are also commonly referred to as the texture of the board. Knowing what the nuts is allows you to realize how close your hand is to the nuts. The closer you are the less likely anyone will have you beat.
For example if you have Qc Qs as your hole cards and the board is [2c 4s 8d Qh Kc] then the nuts is three of a kind Kings and you have a very strong hand with three of a kind queens. If the board instead had been [2h 4h 8h Qh Kc] then the nuts is an ace high flush and your three queens is not nearly as likely to be the best.
Your distance away from the nuts is one way of approximating your current hand strength. Hand strength is what percentage of hands you beat. Poker Academy will calculate hand strength in the sidebar for you, showing you exactly how good your hand is against all other possible hands (this is called unbiased hand strength). Most opponents do not play all possible hands pre-flop--they fold the worst ones. The hand strength takes this bias into account but is just an approximation as you can only estimate what hands your opponents play. You can refer to Poker Academy's calculation to check your own mental approximation of how strong your hand is.