From a legal point of view 20 mph Speed Zones are exactly the same as 20 mph Limits with regard to the speed you are allowed to drive at. Both have a maximum speed of 20 mph. Note that these are different from Home Zones which may have design speeds much lower than 20 mph.
The primary difference between "zones" and "limits" is one of engineering and the requirements for additional signing, etc.
20 Mph Zones
20 mph zones require that no point in the "zone" is further than 50m from a traffic calming feature, unless the road is a cul-de-sac of less than 80m in length.
A 20 mph zone is indicated by special 20 mph zone entry and exit signs.
No additional speed limit signs are used within a 20 mph zone because the traffic calming methods should mean that vehicle speeds are low enough for the signs to be unnecessary
20 mph Limits
20 mph limits are indicated by terminal speed limit signs and repeater signs (usually small signs on lamp posts) at regular intervals.
20 mph limits usually do not have any traffic calming features. However these can be added if required.
In the UK context this is often misinterpreted as meaning physical speed constraints such as speed bumps and speed tables (vertical) or chicanes, road narrowing and build outs (horizontal). In fact it can also include gateways, road markings, hatching, carriageway roundels and rumble devices.
Note that in the EU context Traffic Calming is used as a term for "reducing vehicle speeds" rather than referring to the implementation of any engineering measures. Hence when a European traffic engineer refers to their "traffic calming" as a measure for increasing vulnerable road user safety they merely mean that they have taken steps to reduce speeds, including speed limit enforcement.
20's Plenty For Us promotes the use of community wide default 20 mph speed limits in residential areas. These should be implemented without any form of traffic calming which should only be retrospectively introduced where it is found to be necessary. Arterial and main roads should be set by the local authority with reference to the local conditions.