If you're not sure what an abbreviation or expression in an advert for a flat or room means, the Welcome Centre has put together a list of the common ones:
EBK = Einbauküche (fitted kitchen): Not all apartments in Germany are pre-equipped with a kitchen. Make sure it is mentioned in the advertisement.
Kaution (deposit): You are required to pay a security deposit of up to three months' rent. After you move out and no damage has occurred, you will recive the whole sum back. If you've damaged something willingly or unwillingly, the landlord will keep a certain sum for repairs.
Kaltmiete ('cold rent'): The 'Kaltmiete' is the rent without heating or utilities. Keep in mind that you have to pay extra for gas and other utility costs.
Warmmiete ('warm rent'): This is the rent including some utilities and heating. However, electricity and Internet are usually paid extra. To avoid misunderstandings, ask the landlord which utilities are included in the rent and which aren't.
Ablösung/Ablöse: If the previous tenant bought furniture for the property, you may be offered (or, in some cases, required) to buy this from them.
Abst. = 'Abstand': a surcharge paid to the previous tenant. Technically, Abstand means paying the previous tenant to move out earlier, but the term is often used interchangeably with Ablösung, a payment to the previous tenant for furniture they have bought which remains in the flat after they move out.
NK = 'Nebenkosten' (additional costs): In Germany the majority of the running costs can be passed on to the tenant. These costs include everything from property taxes to the lady cleaning the stairways each week and elevator maintenance (but not repairs). See the housing advice page for more information.
Einfamilienhaus: a house designed for occupation by one family / household.
Mehrfamilienhaus: a house designed for occupation by several families / households.
Sanierter Altbau: an old building which has been refurbished to modern standards.
Hochhaus: a high rise / block of flats.
Barrierefrei: accommodation designed to be accessible to people with restricted mobility. It's not the same as being wheelchair friendly or fully accessible for people with disabilities. For example, there may be shallow steps and thresholds between the inside of the home and the balcony or garden.
DG = 'Dachgeschoss': penthouse or attic floor.
EG = 'Erdgeschoss': ground floor.
OG = 'Obergeschoss': upper floor.
BJ = 'Baujahr': year the house was built.
NB = 'Neubau': a recent building.
Qm2 = 'Quadratmeter': size in square metres.
WG = 'Wohngemeinschaft': a house share or flat share.
Whg. = 'Wohnung': an apartment or flat (as opposed to a house).
Wohnungsgeberbestätigung: a document that the landlord needs to provide (or sign) so you can register as a resident with the local authority (which you must do).
Zi. = 'Zimmer': number of the rooms the accommodation has. In Germany, the bathroom and the kitchen are not counted as rooms. For example, if you are looking for an apartment with a living room, a bedroom, a kitchen and a bathroom, you are looking for a two-room apartment ('Zweizimmerwohnung').
Zweck-WG: people renting rooms in a house together purely to save money. "Keine Zweck-WG" means that people don't rent together just to save money, but also to socialise and become friends with their housemates.
Zwischenmiete (interim lease): The actual renter sublets his or her apartment for the time he/she is not in town (e.g. on a semester abroad). Therefore the apartments are usually fully furnished.
You may also find Expatica's glossary & typical conversation phrases for talking with estate agents useful.