Conditional sentences are sentences in which one thing depends upon another.
I will give all the money to charity if I win the lottery.
The result (giving the money to charity) depends on the condition (winning the lottery). The two clauses are joined by the conjunction if.
The two clauses can be swapped around.
If I win the lottery I will give all the money to charity.
There are different types of conditional sentence.
1) Things that might actually happen
If it snows we will build a snowman.
This type of sentence uses the future tense ('we will').
2) Imaginary things or things that might possibly happen but are unlikely
If David Beckham came to our school I would be very excited.
This type of sentence uses words like 'could' and 'would'.
3) Things that cannot be changed because they are in the past
If I had worked harder I would have passed the test.
This type of sentences uses words like 'could have' and 'would have'.