A HDB flat is government subsidized housing. You must have certain criterias to qualify to be able to buy a HDB flat. For example (out of many criterias), you must be a Singapore citizen or a Permanent Resident and belong to a certain income bracket. This is how the government "takes care" of the lower income groups in the country so that they can afford to own an apartment. The HDB flats do not have facilities such as swimming pools or their own gyms. Usually, just a neighborhood playground for the kids, and a small garden or park where there are benches.
Condominiums on the other hand, are private developments of
property companies and cost a lot more than HDB flats. They all have private swimming pools and gyms, security guards round the clock around the apartment complex, BBQ facilities, playgrounds for the kids, and more recently, spas in the
condo complex itself.