All properties except garages can store items and gear inside them, the differences are the maximum capacity. Barns can also grow certain drugs or potatoes.
Gang bases like the towers, warehouses, offices & barracks must be purchased by someone LVL 4 in a group (Y->Groups) and cost a lot more than other properties as all the group can use it.
You can purchase different upgrades for properties, such as increased storage & break in time, spawn point, ATM, CCTV, etc.
All properties can be broken into as long as the owner (or a member of the group for Gang Bases) is online. If nobody with access is online, your stuff is safe.
All properties need to have their rent paid after purchasing them. You get 28 days free upon purchase but can extend it by maximum 60 days at a time (most you can get is 89/90 days as you can't renew the rent at over 30 days). The rent price per day differs with different properties, with gang bases being most expensive as an incentive for the whole gang to fund it.
Any other questions?