"Condo" stands for 'condominium", i.e. the building is mutually owned.
In the UK it would be the equivalent of each flat owner owning a part of the freehold for the building. So in a condo all owners are mutually responsible for the upkeep of the building as whole.
An "apartment" is a flat, it could also be in a "condo", in theory, but is generally used to refer to rented flats.
Note that in Scotland, we don't really have the freehold/leasehold distinction in the vast, vast majority of cases - instead all flat owners are responsible for the upkeep of the building and there is no separate freehold, through a default set of rules defined by statute which may be overridden by the title deeds for each flat. This is different from the rest of the UK, where the freehold is a separate legal entity that is responsible for the upkeep of most structural elements of the building - although each flat owner may own a share of the freehold in many cases.
In the UK it would be the equivalent of each flat owner owning a part of the freehold for the building. So in a condo all owners are mutually responsible for the upkeep of the building as whole.
An "apartment" is a flat, it could also be in a "condo", in theory, but is generally used to refer to rented flats.