In HTML, things like "rel" and "id" and "href" are called "attributes". The "link" thing is a tag. All of it together is an element.
Elements are made up of tags and attributes, in other words. Now, each attribute must be separated by spaces.
You can use single or double quotes, but make sure they match up. For instance, this is okay:
<link rel='stylesheet' id="screen.css" ... />
However, this is incorrect:
<link rel='stylesheet" id="screen.css' ... />
I think that about does it.