While you might assume that these options require a USB keyboard or other physical connection, this is not always the case. These options can be invoked by holding their respective key combinations immediately when powering on the system however, if you use Apple's Bluetooth keyboard, you could find that the system may ignore these inputs and boot normally. In addition there is Single User mode (Command-S) for a command-line interface, Apple's hardware diagnostics tests (the 'D' key), Target Disk mode (the 'T' key), and holding the Option key will bring up the boot loader for choosing a startup disk. Of these, Safe Mode is perhaps the most common it offers a limited boot environment and can be started by holding the Shift key. Apple offers several alternative boot environments for OS X that can help you troubleshoot problems you might be experiencing, which can be invoked by holding various key combinations at startup.
