This text is left-aligned. Note the ragged edge on the right. This is the standard choice for text alignment, and the one that's easiest to read.You'll see it everywhere on the Web.
This text is right-aligned. Notice how the ragged edge is on the left now, where it's much more noticeable. Right alignment isn't used terribly often.
This text is centered. Centering is common in headings, and even for entire pages (check out www.google.com for an example). Each line is centered separately. You can use this alignment option in conjunction with a wider margin if you want the centering to really stand out.
This text is justified, which means the browser tries to eliminate the ragged edge on both sizes by adding space in between the words. Due to the primitive justification algorithms used by browsers, this doesn't always work out too well. If you have small columns, it can make your text look stretched and unnatural.