Every hood you own generates a certain amount of money. Additionally, you earn many extra troops if you control an entire borough. The boroughs are the larger sections of the map which will either be similarly colored, or clearly seperate. And finally, the longer you control a hood, the more income it will produce for you.
This income, by default, will be used to purchase new guys at the start of your turn at the rate of $300 per guy. No matter how poorly you are doing, you will always receive a certain minimum of guys a turn.
At the bottom right of the screen, a report shows you your income (in guys) broken down into what you receive from all your hoods, and then any extra income you might be receiving for owning an entire borough.
 
In the above picture, Moe Green currently controls 6 guys. The land he controls is generating enough income to purchase an additional 5 guys, and finally, he controls the whole borough of The Bronx -- this is giving him 8 more guys for a total of 13. So at the start of Moe Green's turn he will not have 5, but 19 guys to use in his attempt to become the Boss of Bosses.
For a detailed breakdown of your income and which hoods belong to which borough, click on "Da Breakdown" button at the bottom of the screen.
