There are two different types of car modifications and both are likely to have an effect on your car insurance premium.
An insurance company is likely to view any aftermarket change to the car away from the standard (how it rolled out of the car assembly factory) as a modification.

You must make sure you declare and tell your car insurance company about any modifications you make to your car as if you do not declare them and you are involved in an accident you run the risk of having your car insurance invalidated.

Cosmetic Modifications
Cosmetic modifications include things such as alloy wheels, body kits, spoilers, ICE (In Car Entertainment). Although not necessarily making your car faster they may make it stand out and become more "attractive" to thieves or vandals. This in turn means there is a higher risk to car insurance companies to insure it.

Performance modifications
Performance modifications are pretty straight forward. They are any modification that is going to improve/increase the performance of the vehicle. This includes things such as lowered on new springs/suspension, exhaust, induction kits and any other engine tuning modifications.
The faster the car can go may increase the likelihood of an accident.