In San Andreas, you can find a whole lot of properties which you can own. It may be something connected with activities (like hangar), or some business which will generate every week profit. To buy a property, you have to find it and pay. New properties are unlocked as the game progresses. What is more, some of them are limited to one or two characters, while other may be bought by all of them.
When you become an owner, you'll receive messages about completing a mission from time to time. Ignoring it decreases the profit from particular week. Ignoring it may decrease or even totally destroy the income from the particular week.
Before you buy a property, remember that this is a long-term investment which won't refund if you plan only to finish the game and end your adventure with GTA. One week is about five hours of constant playing (besides sleeping or hospitalizing what speeds up the time). Simultaneously, to get a 100% of game completion, you have to have 5 generating income properties, so they are necessary. There are also an alternative variant of spending your time in San Andreas, so, after all, you should get interested in them. But if you care only about money, you should notice how quickly the investment will refund before you buy it. Compare the general price with the weekly income. The best refund financially is within about a year and a half. You should consider for example Smoke On The Water or The Hen House which are cheap with a solid income.
If you're ready to take things in your own hands, some properties will pay for missions. Waste Dump pays 150$ for every destroyed car while Sonar Collections Dock pays 23 thousands for a portion of nuclear waste (but there are only 30 of them). Focusing in this particular areas you can very quickly make those business return your payments and start profiting faster than in classical model, but it's time consuming.
At the end, there are some businesses, which are financial black holes, but may be useful from other reasons. For example, buying a taxi company, will give you free taxis, what may be useful, when you want to get on the other side of the map and you won't want to spend 10 minutes driving. Buying Los Santos Customs workshop in Grand Senora Desert gives you free car upgrades for Franklin.