Political strife and war are no strangers to the Star Trek universe, with enemies like the Klingons and Romulans to give the Federation trouble, the phaser banks are constantly being used. The difference between them and the Borg though, is that where the Romulans can be reasoned with and the Klingons can become allied, there is no reasoning with the Borg. There is no real alliance with the Borg. There is no peace with the Borg.
The Borg are a species who melded their technological advancements with their own bodies, creating an entire race of cyborgs. They've also linked their individual collectivness into a singular hive mind, making them all think and act exactly alike. Among the Borg there are no individuals, no families, no artists, no musicians, no heroes and no villains. They are just Borg. Their goal is to expand their galactic empire by capturing or killing every other sentient species in the universe and adding their technological advancments, as well as their populations, to their own. They can link any species and any person into their collective making that person Borg. They've been called the zombies of the Star Trek universe, but where zombies are mindless killing machines, the Borg are actually incredibly intelligent, using their vast stolen technology to disarm and defeat any enemy that crosses their path.
The species first showed up in Star Trek the Next Generation, being used by the Q to show Picard and the crew of the Enterprise that the galaxy was too dangerous for humans to continue to explore. Barely escaping with their lives, the Federation plowed on. Probably one of the most important encounters with the Borg happened when they abducted Picard and made him their "liaison" to humanity, Locoutis. The Enterprise was able to rescue the captain and turn him back, but not before the Borg tore through the Federation like a hot knife through cheese.
The storyline carried over into the Star Trek film First Contact, where they actually encountered the Borg Queen, their collective leader. The Borg travel back in time to try and assimilate the Earth long before the Federation is in the picture, and if not for Picard and Data they would've succeeded. It's arguably the best Star Trek movie they've made thus far all because the villain is such a threat.
In Voyager they had to fly through Borg space to get home. During their trip, they picked up a Borg they named Seven of Nine. Formerly human, she was assimilated as a child and has spent most of her life as a Borg. Because of that, she had no idea how to be a human, and became an interesting perspective on humanity through the eyes of the outsider, much like Spock, Data and to a lesser degree Odo were. It didn't hurt that she could fill out a skin tight one piece like nobody else on Star Trek could.
If this post isn't as funny as some of my other Creature Features, it's because there really isn't anything that funny to say about the Borg. They're probably the darkest part of the entire Star Trek Franchise, showing up covered in alien technology with that blank, robotic look on their faces and that cold Mr.Freeze voice. There a frightening and awesome addition to an already colorful and amazing universe.