difference between condensation and dehydration?

Condensation is a class of reaction. In condensation, the two reactants undergo addition and small molecules are eliminated in the reaction.
Dehydration is a kind of elimination in which water molecules are eliminated in the reaction.

In short, condensation is an addition-elimination reaction, while dehydration is an elimination reaction.

For example, water molecules are eliminated in both of the following reactions.
CH₃COOH + CH₃OH → CH₃COOCH₃ + H₂O (H₂SO₄ as catalyst)
CH₃CH₂OH → CH₂=CH₂ + H₂O (heat and catalyst)
The first reaction is condensation because it is addition-elimination, while the second one is dehydration because it is only elimination.

A condensation reaction is a polymerisation reaction where two or more monomer units join together and a small molecule, often water, is eliminated and dehydration reaction is a reaction where water is removed from a compound.