Jeff, Not sure what the analogy is to "extensions" on a PC. Inits maybe? They're kind of like little helper apps. Drivers, Norton File Saver, virus protection, Adobe Type Manager, etc. You can turn them on or off, depending on which ones you need. Sometimes some of them conflict with each other. Many software installations recommend turning them off to prevent problems during installation. Virus protection programs especially can affect installations. Mac has memory "assigned" to each software application for minimum and maximum amounts of RAM. You have the option of setting these yourself. The system won't let you open an app if at least the minimum amount isn't available (say you have 2 or 3 apps open). The system will use as much RAM as is needed to run your app, up to the maximum assigned to it. If you need more than you have assigned, you have to go in and allocate a larger maximum.