As traditionally defined, the true Trilobita are divided into two main groups, the Agnostida and the Polymerida. Agnostids have sometimes been considered non-trilobites -- even crustaceans, by some -- but recent analyses (e.g. Fortey and Theron 1994) support the inclusion of the Agnostida in the Trilobita.
A third group, the Nektaspida, was not identified until the 1980s; its members include soft-bodied arthropods from the Burgess Shale such as Naraoia. Some, such as Gould (1991), have classified the Nektaspida as true trilobites; others (e.g. Simonetta and Delle Cave, 1991) have disagreed. On this cladogram, which is based on the work of Fortey and Theron (1994) and Wills et al. (1993), the Nektaspida are the sister taxon to the polymerid and agnostid trilobites; whether the Nektaspida should be considered "true" trilobites or not is largely a matter of definition.
Many other Cambrian arthropods, especially those from the world-famous Burgess Shale, were once considered probable trilobite relatives, or "trilobitomorphs." While many of these problematic arthropods are no longer thought to be related to the true trilobites, recent studies have shown that a number of them are closer to the trilobites than to any other arthropods. The term "Trilobitomorpha" is used here in this more restricted sense. Exactly how these arthropods are related to the trilobites is not yet perfectly clear, but the non-trilobite "trilobitomorphs" probably compose several clades. One of the clades that probably fits in here is called the Emeraldellida. Yet another group of arthropods, the aglaspids, have traditionally been grouped with the chelicerates, but may be closer to trilobites. Some authors also group the emeraldellids with the cheliceramorphs as well (e.g. Simonetta and Delle Cave, 1991). Whether or not these groups actually belong to the cheliceramorphs, it seems likely that the cheliceramorphs are the closest living group to the trilobites and their relatives.
Simonetta, A.M. and Delle Cave, L. 1991. Early Paleozoic arthropods and problems of arthropod phylogeny, with some notes on taxa of doubtful affinities. In: Simonetta, A.M. and Conway Morris, S. (eds.) The Early Evolution of Metazoa and the Significance of Problematic Taxa. Cambridge University Press, Cambridge.
Fortey, R.A. and Theron, J.N. A new Ordovician arthropod, Soomaspis, and the agnostid problem. Palaeontology 37(4): 841-861.
Gould, S. J. 1989. Wonderful Life. W. W. Norton, New York.
Wills, M. A., D. E. G. Briggs, and R. A. Fortey. 1994. Disparity as an evolutionary index: a comparison of Cambrian and Recent arthropods. Paleobiology 20(2): 93-130.