A surd is a square root that is an irrational number. The number is only exact when left in surd form, if you use a calculator to give you the exact value, it becomes in-exact.

Example of a surd: √3 . If I ask my calculator what √3 is, it'll say 1.73205060757, which has truncated a load of digits. So it's best to manipulate this number in surd form.

Some rules when manipulating surds -

√a * √b = √(a * b)
√a divided by √b = √(a divided by b)