A conditional sentence shows that an action is reliant on something else (there's a condition).
We often come across conditional sentences as children.
|If you don't eat all your vegetables,||you won't get any pudding.|
|If you eat all those sweets,||you will be sick.|
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.
Note! You can swap if clauses around, but if the "if" clause comes first, a comma is usually used, but if the "if" clause comes second, there is no need for a comma.