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From UC Berkeley:
Long classified together in the Myxomycophyta as part of the Fungi, slime "molds" are now known to be quite unrelated to the fungi. There are three main groups of slime molds, which do not form a clade.
Plasmodial slime molds, like Physarum shown here, are basically enormous single cells with thousands of nuclei. They are formed when individual flagellated cells swarm together and fuse. The result is one large bag of cytoplasm with many diploid nuclei. These "giant cells" have been extremely useful in studies of cytoplasmic streaming (the movement of cell contents) because it is possible to see this happening even under relatively low magnification. In addition, the large size of the slime mold "cell" makes them easier to manipulate than most cells.
A second group, the cellular slime molds, spend most of their lives as separate single-celled amoeboid protists, but upon the release of a chemical signal, the individual cells aggregate into a great swarm. Cellular slime molds are thus of great interest to cell and developmental biologists, because they provide a comparatively simple and easily manipulated system for understanding how cells interact to generate a multicellular organism. There are two groups of cellular slime molds, the Dictyostelida and the Acrasida, which may not be closely related to each other.
A third group, the Labyrinthulomycota or slime nets, are also called "slime molds", but appear to be more closely related to the Chromista, and not relatives of the other "slime mold" groups.
What these three groups have in common is a life cycle that superficially resembles that of the fungi. When conditions become unfavorable, these slime molds form sporangia - clusters of spores, often on the tips of stalks such as in the sporangium of a Physarum shown at right. Spores from the sporangia are dispersed to new habitats, "germinate" into small amoebae, and the life cycle begins again. Similarities in the life cycle do not, however, imply close relationship, especially when one considers that certain bacteria (the myxobacteria) and even an unusual ciliate have very similar life cycles, aggregating to form spores on a sporangium.
Slime molds have almost no fossil record, which is not surprising. Not only do slime molds produce few resistant structures (except for spores, which are often overlooked or unidentifiable), but they live in moist terrestrial habitats, such as on decaying wood or fresh cow dung, where their potential for preservation is low. A few fossil slime molds have been found in amber (Poinar and Waggoner, 1992).
Slime mold is a broad term describing fungi-like organisms that use spores to reproduce. They were formerly classified as fungi, but are no longer considered part of this group.
Their common name refers to part of some of these organism's lifecycles where they can appear gelatinous (hence the name slime). However, this feature is mostly seen with the myxomycetes, which are the only macroscopic slime molds.
Slime molds have been found all over the world and feed on microorganisms that live in any type of dead plant material. For this reason, these organisms are usually found in soil, lawns, and on the forest floor, commonly on deciduous logs. However, in tropical areas they are also common on inflorescences, fruits and in aerial situations (e.g., in the canopy of trees). In urban areas, they are found on mulch or even in the leaf mold in gutters. One of the most commonly encountered slime molds, both in nature in forests in the temperate zones of the earth as well as in classrooms and laboratories is the yellow Physarum polycephalum.