The thing you need to know about the buggers is that they’re out to get us, everybody RUN AWAY!
Actually, no, wait, that’s totally wrong. Even though buggers are aliens (scary) that have tried to invade Earth twice (scary), they are not actually trying to hurt humans anymore. They just want to live in peace and they’re very sorry for the confusion that resulted in any human deaths. Basically, the buggers are more forgiving than humans. And we kill them all. Whoops.
Of course, we don’t know they want peace until after Ender wipes them out. (Actually, we only 100% learn it when Ender figures out when he discovers the last remaining bugger, which is a nice coincidence: Oh, you weren’t evil; and I didn’t kill all of you, so we can live in peace together after all. Yay.)
We don’t know very much at all about the buggers. For instance, we don’t know how many there are or what sort of weapons and ships they might have now, since it’s been eighty years since their last invasion attempt.
But there are some things that we do know about the buggers. As Graff explains to Ender, they’re basically smart space ants – which also means that they could only survive on Earth-like worlds. (Which aren’t super common and so must be pretty valuable as real estate. In other words: humans want those worlds because we have this overpopulation problem.)
Let's start with the "smart" bit. How smart are they? Well, Ender recognizes that they use clever tactics when fighting (11.112). This is why Ender turns to the buggers to learn how to beat them (it clearly won’t help him to study the tactics of the humans he keeps beating).
Now you must be wondering how they're like ants. Well, they live in this hive organization (like ants and bees), with only one of them breeding and thinking for the whole hive. In other words, every hive is its own organism and the individual workers aren’t really important as individuals. Interestingly, they also communicate telepathically.
But here’s the big thing: we don’t know (at first) why they attacked humans in the First and Second Invasions. According to Graff, since we can’t talk to them to figure out why that happened, the only logical thing is to wipe them out before they invade a third time (13.280): “If the other fellow can't tell you his story, you can never be sure he isn't trying to kill you” (13.282).
At the end of the book, we learn that buggers are basically better than humans: they’re really sorry about killing us and they only did so when they didn’t realize humans were intelligent (15.174). Hmm…compare that to the fact that we kill them after we knew they were intelligent. And they forgive us for killing them all. The final straw: the buggers only killed a few humans and “never deliberately attacked a civilian population” (14.331), whereas Ender purposefully destroyed their entire planet. Talk about a study in contrasts…
The buggers are basically really nice aliens. What Ender discovers after nearly wiping them out is that the buggers are actually pretty good neighbors. When Ender finds one living "queen" bugger, we’re glad that the buggers can carry on their nice legacy after all.