If your family shares a Macintosh, you can create a separate account for each family member. Although only one person can use the Macintosh at a time, each person can customize his or her own account, storing songs in iTunes, changing screen colors, and storing web page bookmarks.
Even if you're the only person who uses your Macintosh, consider creating separate accounts. One account might contain your important files, while the second account can serve as your experimental account, where you can safely try different commands and experiment with different settings just to see what happens. If you make changes that wreck your computer settings, just delete that particular account, create a new account, and start experimenting with the new account. By creating multiple accounts, you can safely learn different features of your Macintosh without the risk of wiping out data by mistake or totally wrecking your Macintosh's settings and not knowing how to fix the problem.
Creating user accounts essentially splits a single Macintosh into multiple computers, so feel free to experiment and play with your Macintosh using different accounts. Remember that the worst that can happen is that you'll mess up one account beyond repair, but as long as you don't mess up an account that contains your crucial files, separate accounts on your Macintosh can help you learn to use your computer safely and effectively.