NES in a nutshell:
you have 2Kb of onboard RAM. There's an 8Kb window where your cartridge can patch more in. It can even bankswitch to have different sets of 8Kb. You have a 32Kb program ROM window and that too can be bankswitched, usually in 8 or 16Kb sections. Graphics have an 8Kb window, two pages of 256 8x8 tiles. The remaining space in the 64Kb memory is various hardware registers where you do your IO and configuring the graphics and sound generators. There is only one timer available, the TV refresh signal. On an NTSC NES it will raise an interrupt 60 times a second, and on a PAL one, 50.