A condominium is a multi-unit building, ranging from townhomes to high-rise apartment complexes. Each of the units is purchased, and owned by, individual tenants, in contrast to a regular apartment building, where the tenants pay regular installments of rent and do not own anything beyond the contents of their unit. Tenants share ownership of the common areas. Their mutual interest is typically represented by an elected management board.
A condominium is usually (but not always) more upscale than its apartment block cousin - with a 24 hour concierge, a swimming pool, squash courts, an exercise room, and so on.
Condominium owners have the joys of home ownership - market volatility, taxes, utility bills, repairs, and so on - mixed with the joys of living in a multi-tenant building - noise, abuse of shared resources, dangerous underground parking, pure density of humans. Not to mention monthly "condo fees" used to pay for common services and repairs.
On the other hand, condo owners are free from the bother of exterior maintenance and repair, don't have to shovel snow (where applicable) or mow the lawn, and can afford to live closer to prime real estate locations than ownership of a private home might allow.