Macs can easily read PC-formatted hard disk drives. Writing to them, however, is a different story.
If you've switched to the Mac, welcome aboard. Your old external Windows PC drive will work great on the Mac. Apple has built OS X Yosemite and some previous OS X releases with the ability to read from those disks just fine. If you're using such a drive and you'd like to write new data to them, you'll find you can't unless you add new software. Fortunately, you don't have to spend a dime.
One option is to back up your old Windows external drive (using Time Machine or another method). Reformat the drive using Apple's Disk Utility software and the company's HFS+ file system instead. Then you can restore the backed up data to the drive.
Even if the backed up and restored files originally came from a PC, they'll be stored on the drive using a file system the Mac fully understands. That way the drive will be fully Mac-compatible without any need for you to modify the operating system of the Mac to get it to work properly.
Obviously that solution doesn't work for everyone. Maybe the drive you're using has to be used with a PC occasionally. Whatever the case, the good news is that it's not a show-stopper: There are a few utilities out there that will enable Macs to write to mounted NTFS volumes.
Tuxera's NTFS for Mac is one of the best ways to do it. It uses smart caching to keep data transfer as fast as possible and works with every OS X version since 10.4 (Tiger). NTFS for Mac costs $31, and you can download a demo first to see how it does.
Paragon Software's NTFS for Mac 12 is another excellent choice. It includes several additional utilities for people who need to tinker or repair, to enable you to format drives with NTFS, check NTFS partition integrity, fix errors, and more. NTFS for Mac costs $19.95.
If you're a DIYer and you'd like to go the free route, you'll find a Sourceforge project called NativeNTFS-OSX that gets the job done. NativeNTFS isn't for rookies: It's a bash script that needs to run from the Terminal command line and requires you to have root (administrator) access to your computer.
An easier way to go is to download OS X Fuse, a third-party software tool that extends the Mac's file system capabilities. Follow the directions on the OS X Fuse website to download and configure the software. Follow the instructions to download NTFS-3G for Mac OS X, whose development seems stopped right now but still works in Yosemite. Once OS X Fuse and NTFS-3G are installed, your Mac should be able to read and write to NTFS disks just fine.