An argument from analogy is an argument that has the following form:

Thus, for example, a few years ago one Republican congressman, who had been a fighter pilot during the Vietnam War, argued in a caucus prior to the election of the Speaker of the House:

One evaluates such an argument by examining the analogy. It is a weak analogy, and thus fallacious, if there are not many similarities. For instance, in this example there is some similarity between the two situations. The Congressman no doubt felt that with Speaker Gingrich having been charged with ethics violations that he was under attack as a fighter pilot's wingman could be. But there are also dissimilarities. Voting for Speaker of the House is not a life-or-death situation. Moreover, n combat, one neither gets to choose one's wingman nor one's mission. Yet it is the obligation of a congressman to vote for the officers of the House of Representatives as s/he sees fit.

Here's a stronger analogy:

Notice the form is the same for a weak or a strong analogy. What makes a weak analogy fallacious is not the pattern of reasoning but a lack of compelling similarities to warrant the alleged one.