The three Johannine epistles are all anonymously written, as was the gospel of John. No one knows who any of these "Johns" were, but none of them was who believers have always believed him to be: the apostle John, "the beloved disciple." The three epistles and the Gospel of John share enough features, however, that they are thought to have been written by members of the same community, and the three Johannine epistles may have been written by the same non-John.
Here are some things you should know from First John:
Whoever says he is without sin is a liar (1:8) and Christians don't sin (3:6). So Christians lie if they say they don't sin (since everyone sins) and they lie if they say that they sin (since Christians don't sin). Which means, I guess, that all true Christians are liars.
The "John" of First John knew he was living in the last days since there were so many antichrists around at that time (2:18). But since antichrists are people that don't believe that Jesus is the Christ (2:22), all non-Christians are antichrists. So there must be somewhere around five billion antichrists alive today (depending on whether you count Catholics or not), which is a heck of a lot more than there were in "John's" day. Which means we are living in the latter day last days. Or something like that.
Here are some more highlights.