A conditional sentence shows that an action is reliant on something else (there's a condition).
The two most common conditionals are real and unreal, they are sometimes called if-clauses.
The real conditional (often named 1st Conditional or Conditional Type I) describes situations based on fact.
The unreal conditional (often named 2nd Conditional or Conditional Type II) describes unreal or imaginary situations.
There is also what we call the 3rd conditional (often named
Conditional Type III), used to express no possibility of something
having happened in the past, and the 0 conditional (often called the
zero conditional) is used to express absolute certainty.
Unless you are studying English to pass an exam or test don't
try to remember the types, just learn the structure so that you know how
to express the meaning conveyed by each type - it's going to happen -
it's only going to happen if something else happens - it's never going
to happen.