Now that you have your PCs (or Macs) and a server or NAS, you’ll want to join all this stuff together and get online, and this is where a network comes in. Once upon a time this would have been a complex task, but today it truly is plug’n’play (or in the case of wireless, just “play”).
When you sign up for Internet access with your ISP, you’ll probably have an option to upgrade from the standard bundled modem to a router at a discounted price. This is a pretty good option as most will have four (wired) network ports plus wireless. If you need something with more ports or more features then have a look at products from Netgear or Linksys.