Addition Polymers add dozens of the same monomer together to make a long molecule with a very simple repeat unit. The monomer contains one functional group (reactive part of the molecule), which is always a C=C double bond.

Condensation Polymers (usually) have two monomers that react together forming a long molecule with a more complicated looking repeat unit.

They also produce a small molecule that is often water - hence the term Condensation.

This isn't a polymer because when one alcohol molecule joins one acid molecule the ester is complete.

But it is possible to make molecules with alcohol groups on either end - diols.

These could form an ester at either end, and we could combine them with molecules that have two carboxylic acid groups.

This combination could form a long chain, forming some water at the same time.

This is a Polyester - a condesation polymer.

But, given monomers like the ones above it would be a good idea to be able to draw the polymer by linking them together and removing water.

If we're asked to draw the repeat unit of a polyester it will always follow the pattern above, the only part that will change is the parts of the molecules between the functional groups.

For example

or