These are the three most common versions of poker:

The most common home version is called _5-card draw_, and is played as
follows:  each player is dealt five cards.  there is a betting round
beginning with the player to the left of the dealer.  Once the betting
round is complete, all remaining players are asked if they wish to
discard any cards, again in a clockwise order beginning to the left of
the dealer..  The maximum number they may discard varies from 3 to all 5
depending on the house rules, but there is never an obligation to discard
any.  The discarded cards are then traded for the same number of new
cards.  Once all players have been given the oportunity to discard cards,
a betting round ensues, and after that, a showdown.

The most common casino version of poker is "7-card stud".  In _7-card
stud_, you are dealt two cards face down, then an additional card face
up.  There is a betting round and all remaining players are dealt three
more cards face up, with a betting round in between each deal.  After the
4th face up card has been dealt, there is a betting round, then one more
card is dealt face down, and then there is a betting round and a
showdown.  While the player has 7 carrds in his possetion, only a subset
of 5 of them constitutes his "hand".  The version is called _stud_
because no cards are exchanged at any point (and you can see some of the
other players' hands).

The other most commonly seen casino poker game is "Texas Hold'em".
_Texas Hold'em_ is played by having each player dealt two cards (face
down).  Then there is a betting round.  Then three "community" cards are
dealt face up in the center of the table.  A _community card_ is a card
that can be used as a part of any player's hand.  There is then a betting
round, then another community card is flipped, then another betting
round, one last community card is flipped, then a betting round, then a
shodown.  Using the 5 exposed cards in the middle of the table and the 2
in their possetion, players form the best 5-card hand they can.