A co-op is (usually) a non-profit that owns an operates a building. When you buy a co-op apartment you effectively buy a share(s) in the co-op which entitles you to occupy a particular apartment. As a shareholder in the co-op you are required to pay maintenance to cover your share of the operating costs of the building: energy and utilities, maintenance, doorman etc. Some co-ops also generate revenue from flip taxes.
Maintenance doesn't have to equal the mortgage payment - our Brooklyn co-op costs around $650pcm in mortgage, and $800 in maintenance.
I have a share in the freehold of my flat in the UK, my maintenance comes in at £480 per annum, this includes buildings insurance, cleaning, heating and general building maintenance.
A good whack of it goes towards fuel oil to run the steam heat system, then you've got a surgeon sized insurance premium in case some litigious oaf slips on the sidewalk, and finally a couple of full time staff.
Maintenance doesn't have to equal the mortgage payment - our Brooklyn co-op costs around $650pcm in mortgage, and $800 in maintenance.