Wireshark uses dissectors to figure out what protocol a given frame is. When you look at the "rainbow" that is windows wireshark traffic you will see a myriad of protocols flying by that you had no idea existed, and you will learn quickly that windows machines are very lonely and want to make friends on a network. A great way to learn about all these protocols is just to go to wikipedia and research them. For example, on your screen, you could research pretty much any of those protocols shown (ARP, DNS, ICMP) and learn alot about what your computer is doing in the background.