You can compress and decompress .zip format archive files using Windows' own built-in capabilities, but that's limited. Or you can use a commercial program like WinZip that's packed with all sorts of goodies, but you have to pay for it. Or you can use 7-Zip, which gives you a couple of the most important features for free. It's available in 32-bit and 64-bit versions.
For instance, 7-Zip can compress and decompress WinZip-compatible AES-encrypted .zip files. That's vital if you need to e-mail sensitive data, because the .zip format's standard password protection (the only kind Windows supports) is easy to crack. But with 7-Zip, you can share truly-secure data with people who use WinZip, PKZip, WinRar, PowerArchiver, and other programs. And if they use none of those programs and don't want to spend money, you can tell them to get 7-Zip.
7-Zip also allows you to compress and e-mail files in one easy step. Just right-click a file or files in Windows Explorer and select 7-Zip¸ then Compress and email. It even has its own compression format, although I recommend sticking to .zip for compatibility purposes.
This isn't to say that 7-Zip will satisfy every WinZip fan. It can't compress a .zip into an .exe. And the user interface is plain, dull, and unattractive. It can also be intimidating, offering options that most users should never have to think about.
But you don't have to. The default settings are just fine. For most compression tasks, 7-Zip does the job.