Insurance is offered by many casinos.  When the dealer gets an ace up, the player is
asked if they want insurance.  If insurance is taken the player must put up half of the
bet as insurance.  If the dealer has a ten the player gets all of the bets back.  If the
dealer does not have a ten, the house takes the half insurance bet.  If the player has a
blackjack and the dealer has a ten, the player gets even money, instead of the usual 3 to 2
blackjack bonus.

There are 36 non-tens and 16 tens in a single deck of cards.  This is a ratio of 2.25 to
1.  Insurance pays 2 to 1 since it is simply a side bet of whether the dealer has a ten
with an up ace.  Therefore the house has a pretty big advantage on insurance.  Because 
of this insurance is not recommended, unless you are counting cards.  Unfortunately 
all of the books I have read and web sites have different advice on insurance and when
to take it when counting cards.  Personally I never take insurance unless the count
is extremly in my favor.