They are in a war with the magical Moon Clan of Lunar Isle, because of a difference of opinion when it comes to Runecrafting. The Fremennik do not approve of the crafting of Rune stones, and think that humans should only use the runes they were given by the gods. The hatred between the two clans is lessened during the Lunar Diplomacy quest.
Players become an honorary Fremennik after having completed the Fremennik Trials quest. Then upon completion of the trials, the player is given a Fremennik name (however, if the player is wearing a Ring of charos (a) they may change their name at random by speaking to Brundt the Chieftain). Players may speak with the villagers and find out a lot more about them. The purchase and wearing of Fremennik armour is also made available after the trials.
The Mountain Tribe, were originally part of the Fremennik too (much like the Moon Clan), until a dispute tore them apart.
The Fremennik have their own language, which can be discovered when examining the safety guarantee used in the quest Mountain Daughter. This document examines as: "The strange characters supposedly grant Svidi (a representative of the Mountain Tribe) safe passage into Rellekka." The strange characters must be the writing of the Fremennik.
The Fremennik are direct descendants of the humans Guthix brought with him from the original human realm when he was populating Gielinor in the First Age. They were once a nomadic tribe. They lived a simple life, hunting the local animals, fishing, and travelling throughout the Fremennik Province so as not to exhaust the land's rich resources.
Eventually, they abandoned their nomadic way of life, creating the village of Rellekka in the Fourth Age. While this greatly improved life for most Fremennik, some disagreed with this way of living. Many left the tribe, grouping together to form the Mountain Tribe, to continue the traditional nomadic lifestyle. This angered the Fremennik, and the two tribes have been somewhat bitter towards each other ever since, until the player eases these tensions slightly during the Mountain Daughter quest.
Some years ago, the Fremennik discovered a rock that was revealed to be Rune Essence. This caused a great struggle within the tribe; some believed the Fremennik could harness the magics of the essence to defend themselves from the races that were constantly seeking to conquer their land. Others believed that the rock was not to be used by mortals, and that the gods would destroy them if they used it. The tension grew until many more Fremennik departed for Lunar Isle, where they would form the Moon Clan.
Taking advantage of the Fremennik's temporary weakness in numbers and absence of awareness, the Mountain Tribe attacked the village of Rellekka and stole the rune essence rock which they believed to be sacred, taking it back to their encampment. The two tribes avoided contact from that point on, until the Mountain Daughter quest.