Like the single player mode of Grand Theft Auto 5, there are numerous properties scattered around Los Santos available for purchase in Grand Theft Auto Online. (See the list of properties available in single player mode here).
In its current form, there are two types of properties you can buy in GTA Online: Apartments and Garages.
As the name suggests, owning an apartment in GTA Online will grant similar functionality as houses in the single player mode. The apartment basically functions as a place to hangout. You can watch TV, invite friends over to hang out and chat, and store cars. Apartments are also safe from PvP.
Purchasing a property will grant you with "The American Dream" Achievement/Trophy
Unlike Apartments, Garages in GTA Online are designed solely for storing your vehicles. Cars stored in a Garage are fitted with a Tracker, which protects the vehicle from permanent theft by thieves. If a thief does steal your tracked vehicle, it will appear on the map and players can create a bounty for that stolen car.
Purchasing a Property
Properties can be quite expensive so be sure to save up an adequate amount before buying one. Once you have enough money, you can purchase properties for sale through the in-game internet accessible through your Cell Phone, or through an in-game terminal.
Players are only allowed to own one property at a time, whether it be a garage-only or an apartment with an attached garage. If you decide to start small and upgrade later, half of the value of your old property is applied as "trade-in value."
If there is an equal or greater amount of parking available in the new property, all of your saved vehicles will be transported automatically and instantly for no charge.
Complete List of Properties For Sale in GTA Online
Below is a comprehensive list of properties available for purchase in Grand Theft Auto Online. These were provided by Reddit user pvt13krebs.