There are literally loads!
Circular signs that are RED tell you what you must not do.
For example:
You must not exceed 30mph
No vehicles over the height shown may proceed
No pedestrians
No motor vehicles
If the circle is BLUE, generally they give a mandatory instruction, such as "turn left", or indicate a route available to particular classes of traffic, e.g. buses and cycles only.
Triangular signs give warning of things that are coming up.
For example:
GIVE WAY
Staggered Junction
Roundabout
Crossroads
Traffic signals ahead
BLUE background signs are used for information signs except on motorways where blue is used for direction signs.
GREEN background signs, are used for direction signs on primary routes.
WHITE background signs are used for direction signs on non-primary routes, or for plates used in combination with warning and regulatory signs.
The reason behind marking these signs more prominent are so they can be easily recognised earlier and should they become covered in snow for example.
The GIVE WAY sign is the only triangular sign that points towards the floor, and the STOP sign is the only sign that is Octagonal