Money is generated by buildings and by clicking on the big coin at the center of your town.
The main purpose of money is to spend it. Spend it on new buildings or upgrading existing buildings so they will generate more money for you.
Besides your current amount of money your overall money gained will be tracked aswell.
You can see this by clicking the Milestones monument just above the big coin.
Milestones are special checkpoints, reaching one will give you a bonus when you reset the game. The more milestones you reach the bigger your reward.
You can find more on milestones on the milestones page.