The UK sign system
Each type has adifferent shape. A further guide to the function of a sign is its colour.
All triangular signs are red.
Circles give orders
Blue circles generally give a mandatory instruction, such as "turn left", or indicate a route available only to particular classes of traffic, e.g. buses and cycles only.
Red rings or circles tell you what you must not do, e.g. you must not exceed 30 mph, no vehicles over the height shown may proceed.
The words "must" or "must not", when used in the descriptions that follow, refer to legal requirements that have to be obeyed.
Wikipedia gets into some amazing detail on road signs if you want to delve further into this.