What differences are there between a short term apartment and a hotel room?

First of all: Presentation.

When you book a hotel you usually see pictures of the nicest room. When you book an apartment, what you see is what you get.

Apartments mean economy.

No room service, making a trip to the local grocery store will provide you everything you need for several days at a fraction of the cost in your very own kitchen. Food for more than 2 in a restaurant can easily cost 100 euro per meal. That goes for the cleaning too. Apartment providers usually only clean if you ask them too at an extra cost. This helps keeps your cost down.

Apartments are usually significantly cheaper.

Especially for more than one guest, and the savings really grow when groups of 4 or more stay.