Ask HN: How did WannaCry get onto computers?

After the recent so called 'cyber attacks' of WannaCry, I was careful to update any Windows machine I have and install things like EMET and MalwareBytes on them. I switched to Linux years ago because I've heard nothing but bad news concerning Windows, but one thing struck me about the WannaCry infections: I heard the attackers used an exploit pulled from the recent ShadowBrokers leak, something related to 'SMB'. A few questions:

Explain it to me like I'm five please

1.) What is SMB? And is it easy to remove from systems by simply uninstalling it (like I have done[0])?

2.) Does WannaCry just land on a machine through a simple point-and-click exploit? Do they just enter a vulnerable IP address and they can plant the exploit on the machine and run it?

3.) I am aware that it also gets onto machines by people randomly clicking on shady e-mail attachments, but I am very curious about how it simply lands on computers with very little or no user stupidity at all?

[0] I uninstalled SMB by going to > Add or remove programs > Remove windows features