If you ever use the tabs in your web browser, you may have a hard time navigating between them as you take your hand off the keyboard, put it on your mouse, click on the tab you're looking for, look at that page for a moment and then click back on the first tab again. As a person who has loads of tabs open all the time for researching all kinds of things, going back and forth like this is pretty inefficient.
Luckily, two major browsers for the Mac (that is Firefox and Chrome) offer a very handy method for going from tab to tab: Pressing Command and any of the number keys will take you to that tab. So Cmd+3 will take you to the third tab across, Cmd+9 to the ninth, etc. (although in Chrome, Cmd+9 will take you to the last tab, no matter what). This makes it easy to go from tab 2 to tab 7, for example. In Safari, however, Cmd+1 will take you to the first item in the Bookmarks Bar, located under the URL and Search bar. [Updated with some corrections, thanks commenters! - Ed.]
Now for people who have way more than nine tabs open (like me), it's also a good idea to learn the keyboard commands to flip to the next or previous tab in your browser of choice.
Firefox, Chrome: Command+Option+Arrow (left or right)
Safari: Command+Shift+Arrow (left or right)
If you want your tabs in a different order, you can drag the tabs around to rearrange them however you'd like, and then use the keyboard commands to flip between them.
You don't have to learn all the available keyboard commands all at once, but if you can add a couple here and there, you can do a lot to streamline your workflow.