This term particularly refers to users who post comments on articles which not the kind of articles which require any analysis. The articles generally come in 2 types: (1) articles about superficial topics (such as: entertainment, celebrity) that aren't important enough to comment on. And (2) articles where the general opinion about the article is obvious so there is no need to state it.
Comment cattle are usually the kind of people that like to hear themselves talk, and are generally less intelligent than the average internet user. One of the ironies about comment cattle is that they usually post comments on articles to point out how stupid other people are.
Some savvy internet users now create viral stories to draw in comment cattle, such as the girl who supposedly got a tattoo on her arm after dating her boyfriend for only one week - even though it turned out to be a hoax. Even some legitimate news sites write articles which are particularly designed to bring out comment cattle - "The Huffington Post" and "Yahoo" are two examples.
Commenter #1: "This article is stupid. Why do they even publish this?"
Commenter #2: "Because it brings out the comment cattle. Like you."
EXAMPLE 2 - An article about a mom who sold her kids for a Big Mac:
Commenter #1: "Burn in hell."
Commenter #2: "I can't believe a mother would do this."
Commenter #987: "She doesn't deserve to be a mother!"