If you purchase an external drive for your Mac, you might be doing so to free up space on your boot drive, or to provide a central and unrestricted area for all user accounts to access. While this is a common and often recommended practice, if you simply purchase an external drive, plug it in, and start using it, you might be overlooking a detail that could result in slower performance or even data loss.
When you purchase an external hard drive to use with your Mac, it is likely going to be formatted with a FAT32 file system. Since this file system is the one most supported by major operating systems, this allows you to quickly attach the drive to most systems and be able to read and write your data to it.
Apple's support of the FAT32 format is good, and will suffice to allow reading and writing to thumb drives and others that you regularly share between Macs and PC systems, but its support is primarily for compatibility with temporary storage. This means that it primarily serves to allow quick access to a drive you are going to attach and then detach, as opposed to one you are going to keep connected at all times.
For the most part, Apple's support for FAT32 is good enough to allow for continual use of a permanently attached drive; however, there may be times when certain programs experience performance impacts or even data loss when managing central libraries and databases that you might store on the external drive. For instance, Apple has outlined that iPhoto may experience data loss if you store its library on such a drive. Similar behavior might also occur with Aperture, iTunes, and other programs that have central libraries.
As a result, if you are using the drive solely with your Mac, then before use you might want to consider formatting the drive to Apple's native HFS+ (Mac OS Extended, Journaled) format for optimal performance and compatibility.
To set up an external drive for the first time, attach it to your Mac and then launch Disk Utility from the Applications > Utilities folder in OS X. With the program open, select the drive device -- this will be listed above any volume names, and likely have a name that includes details like the make, model, and sizes. With the device selected, a Partition tab will appear, so select this and then choose "1 Partition" from the drop-down menu. Next, click the "Options..." button below the partition scheme layout, and be sure "GUID" is chosen as the partition scheme to use. With this complete, the rest of the options should be set by default for Apple's Mac OS Extended (Journaled) format, so you can click Apply to make the changes.
This procedure will erase all data on the drive, and set it up with the most compatible file system structure for your Mac. Now simply name the drive what you want, and you should be good to go.
If your external drive already has data on it, then you can first check to see if it is HFS+, by opening Disk Utility and selecting the volume name in the sidebar. Upon doing so, at the bottom of the Disk Utility window you should see the Format option show "Mac OS Extended (Journaled)" if it's properly formatted. If not, copy your data off the drive and follow the procedure above to format it.