For the most part your Mac is worry-free and maintenance free. However, glitches and problems do occur sometimes. One of the best ways to get rid of any problems is to reboot your Mac. A simple restart is good enough most of the time. If you have a more specific problem to resolve, you might also want to boot up your Mac in a specific manner.
Pressing Command-R during the startup will allow you to go directly to Lion Recovery. Lion Recovery is a new feature found only in the latest OS X edition. Lion Recovery has all the tools you need to troubleshoot your Mac from reinstalling Lion to repairing your disk. If you have a bootable disk that contains the installation for Lion, you can boot up from said disk by pressing C during the startup.
CDs and DVDs are sensitive things. Once optical media get scratched, you can run into all sorts of problems. A very common problem is that the disk gets stuck inside the slot and cannot be ejected. The easiest way to remove the stuck disk is to hold the mouse or trackpad during reboot until the slot ejects it. You can also press the eject arrow or F12 during the reset.
Some people have different disk drives associated with their Macs. Some of these disks are startup volumes ad you can boot your Mac with these. To choose which startup volume you will be using press the Option key during restart until the Startup Manager appears. Choose between the startup volumes by toggling the left and right arrow keys and press Return once you have chosen.