What is the difference between third-party and comprehensive car insurance?
The difference is whose losses are covered, and the names describe it fairly literally.
Third-party only (TPO) is the legal minimum in the UK. It covers injury and damage you cause to other people — their vehicle, their property, their medical costs — and nothing of your own. Crash into another car and their repair is paid; yours is not. Your own injuries are not covered either.
Third-party, fire and theft (TPFT) adds two specific perils affecting your own vehicle: fire damage and theft. It still pays nothing toward accident damage to your own car.
Comprehensive covers all of the above plus damage to your own vehicle, including in accidents where you were at fault, and typically single-vehicle incidents where no one else is involved. It usually bundles extras that the cheaper tiers omit — windscreen cover, personal belongings, courtesy car, personal accident benefit, and often driving other cars third-party (though this last one has been withdrawn or restricted by many insurers, so never assume it).
The counterintuitive part: comprehensive is frequently cheaper than third-party only. This is not a pricing error. Insurers observe that drivers who choose the bare legal minimum are, statistically, a higher-risk group — often younger, in higher-risk vehicles, or more price-driven. Rating reflects the group, so the more limited product can carry the higher price. Always get quotes for all three tiers, because assuming third-party is cheapest costs people money routinely.
Where TPO still makes sense is a low-value car where the maximum possible payout on your own vehicle is small — though check whether comprehensive is genuinely more expensive first.
Note that no tier covers mechanical breakdown or wear. That is a separate product.