The Arlians are generally a peace loving people. They play a small part in the story as a whole, but make an important point in the Saiya-Jin saga and give an interesting and in-depth look into Vegeta's early character.  Vegeta orders Nappa to "stop and stretch our legs for a bit, get some exercise", Nappa replies "I hope these Arlians put up a fight, haven't had any real competition in a long time.  On landing on the planet, Nappa observes "its a dump" (and for once, I think I agree with him).  Vegeta and Nappa discuss that this planet had an A rating (or something), indicating that it was once a very valuable planet.  Vegeta says "it's surprising what can happen in 3 years", which pretty much proves that something really bad happened to the planet in that time, but we don't know whether there was a problem such as this on the planet before this time.  The 2 are greeted by some inhabitancies of the planet.  They are riding on what look like giant elephant like animals with mammoth like tusks.  The Arlians themselves resemble human sized grass hopers, most of them are gray in color, but some are blue and one is (a female) is pink.  Their voices sound like they are talking under water (I think they are supposed to sound like insects or something-but it gets a bit annoying after a time).  They have big round eyes which glow red when they are angry.  Nappa obviously doesn't like them very much, "stupid looking cricket thing" and "the cricket brigade" are just some of the descriptions used by him.  Vegeta and Nappa decide to have a little fun while they are on the planet, and allow the Arlian fighters to "capture" them, which lulls them into a false sense of security.

The new king is said to be ruthless and has imprisons most of the surviving Arlians.  We are introduced to an Arlian who I believe was someone of impotence before the reign of the current king began, he is awfully kind to both Vegeta and Nappa.  The Arlian, Atla, tells the 2 Saiyans that he had his wife Lemlia stolen from him on the day they were married.

The Arlians wear amour and fight with swords and shields, all the Arlians we see appear to be dressed as warriors, but they do not seem to be experienced fighters and appear to rely heavily on their swords for fighting, there is no actual martial arts (not counting Kendo or other form of swordsmanship).  In MY opinion, this indicates that they have not for long been a warrior race, but they have learned their few skills quickly.  Vegeta and Nappa are "forced" to fight in a gladiator like tournament against the existing champion, Kazar.  The Arlians are nowhere near strong enough to beat or even harm Vegeta and Napa.  After they have a bit of 'fun', Vegeta kills the King and his royal guards who had attacked them.  The prisoners escape and realize there had been a battle in the courtroom.  They see that the king and his men are dead and Vegeta and Nappa are hailed as heroes and are told that they have freed the people from centuries of bondage and will always have a home on Arlia.  

A happy ending, Lemlia and Atla are reunited for the first time since their marriage, and along with the rest of the survivors, are free to do what they like and live their lives.  Needless to say, everyone is very happy about this.  Nappa and Vegeta fly off into space, but just out side of the planet orbit, they stop and get out of their space pods.  Nappa seems a little upset at Vegeta because "that guy said we were their heroes", but Vegeta just shrugs him off and says "heroes aren't all they're cracked up to be".  With that, he effortlessly destroys the planet and fly off in the direction of earth.

The part the Arlians play in the series is more to set the scene as to the predicament in which the earth is in, and to show to the audience who doesn't yet know, what Vegeta could do when he reached earth and why it is so important that Goku beats them both.