There are 52 cards in the pack, and the ranking of the individual cards, from high to low, is ace, king, queen, jack, 10, 9, 8, 7, 6, 5, 4, 3, 2. There is no ranking between the suits - so for example the king of hearts and the king of spades are equal.
A poker hand consists of five cards. The categories of hand, from highest to lowest, are listed below. Any hand in a higher category beats any hand in a lower category (so for example any three of a kind beats any two pairs). Between hands in the same category the rank of the individual cards decides which is better, as described in more detail below.
This is the highest poker hand. It consists of ace, king, queen, jack, ten, all in the same suit. As all suits are equal, all royal flushes are equal.
Five cards of the same suit in sequence - such as 
J-10-9-8-7. Between two straight flushes, the one
containing the higher top card is higher. An ace can be counted as
low, so 
5-4-3-2-A is a straight flush, but its
top card is the five, not the ace. The cards cannot "turn the corner":
4-3-2-A-K is not valid.
Four cards of the same rank - such as four queens. The fifth card can be anything. Between two fours of a kind, the one with the higher set of four cards is higher - so 3-3-3-3-A is beaten by 4-4-4-4-2. It can't happen in standard poker, but if in some other game you need to compare two fours of a kind where the sets of four cards are of the same rank, then the one with the higher fifth card is better.
This consists of three cards of one rank and two cards of another rank - for example three sevens and two tens (colloquially known as "sevens on tens"). When comparing full houses, the rank of the three cards determines which is higher. For example 9-9-9-4-4 beats 8-8-8-A-A. If the threes of a kind were equal, the rank of the pairs would decide.
Five cards of the same suit. When comparing two flushes, the highest
card determines which is higher. If the highest cards are equal then
the second highest card is compared; if those are equal too, then the
third highest card, and so on. For example 
K-J-9-3-2 beats K-J-7-6-5 because the nine beats the
seven.
Five cards of mixed suits in sequence - for example Q-J-10-9-8. When somparing two sequences, the one
with the higher ranking top card is better. Ace can count high or low
in a straight, but not both at once, so A-K-Q-J-10
and 5-4-3-2-A are valid straights, but
2-A-K-Q-J is not. 5-4-3-2-A is the
lowest kind of straight, the top card being the five.
Three cards of the same rank plus two other cards. When comparing two threes of a kind the hand in which the three equal cards are of higher rank is better. So for example 5-5-5-3-2 beats 4-4-4-K-Q. If you have to compare two threes of a kind where the sets of three are of equal rank, then the higher of the two remaining cards in each hand are compared, and if those are equal, the lower odd card is compared.
A pair is two cards of equal rank. In a hand with two pairs, the two pairs are of different ranks (otherwise you would have four of a kind), and there is an odd card to make the hand up to five cards. When comparing hands with two pairs, the hand with the highest pair wins, irrespective of the rank of the other cards - so J-J-2-2-4 beats 10-10-9-9-8 because the jacks beat the tens. If the higher pairs are equal, the lower pairs are compared, so that for example 8-8-6-6-3 beats 8-8-5-5-K. Finally, if both pairs are the same, the odd cards are compared, so Q-Q-5-5-8 beats Q-Q-5-5-4.
A hand with two cards of equal rank and three other cards which do not match these or each other. When comparing two such hands, the hand with the higher pair is better - so for example 6-6-4-3-2 beats 5-5-A-K-Q. If the pairs are equal, compare the highest ranking odd cards from each hand; if these are equal compare the second highest odd card, and if these are equal too compare the lowest odd cards. So J-J-A-9-3 beats J-J-A-8-7 because the 9 beats the 8.
Five cards which do not form any of the combinations listed above. When comparing two such hands, the one with the better highest card wins. If the highest cards are equal the second cards are compared; if they are equal too the third cards are compared, and so on. So A-J-9-5-3 beats A-10-9-6-4 because the jack beats the ten.