This gives away details from GregBear?'s book Eternity (the sequel to Eon).
Read no further if you'd rather read the book.
The Jarts are an alien species first named in Greg Bear's book Eon. They are inhabitants or at least fellow users of The Way, an infinitely long tube construction stretching through space and time. In Eon, they are never really described, but merely a faceless warlike nemesis, locked in battle with the Hexamon and apparently all of humankind. They are a sort of off-screen enemy in Eon, the menace that pressures the gate openers along the way to hurry their work before the Jarts come and destroy them.
In Eternity, the character Olmy Ap Sennon comes face to face and mind-to-mind with a captured Jart and comes to learn their motivations. They are in a sense a sort of Borg, though instead of assimilation, their goal is to archive and preserve records of all places and species to present to Descendant Command, a supreme being at the apex of their rigidly hierarchical society. This preservation is in the form of being recorded and frozen into enormous memory banks, with all records essentially unchanging in the ultimate great library. The Jarts consider the very purpose of their existence to capture and record everything possible in order to present it as a sort of offering to Descendant Command at the time of the ultimate end of the universe. Their antipathy toward humanity is founded by their subgoal of the destruction of The Way; although The Way serves as a means to carry out their ultimate mission, its existence ultimately threatens the teleological purpose of the universe itself, explained as a sort of apocalyptic self-actualization process.