The overwhelming majority of described fungal species are members of the subkingdom Dikarya (Hibbett et al. 2007), which is composed of the two phyla Ascomycota and Basidiomycota. Ascomycota is the largest phylum within the kingdom Fungi, with around 65,000 described species (Kirk et al. 2008).
Ascomycota, which includes both unicellular and multicellular forms, is divided into three monophyletic subphyla:
1) Taphrinomycotina (which includes, among others, Pneumocystis jirovecii, a fungus that is often present in the lungs of healthy people but can cause pneumocystosis in individuals with weakened immune systems);
2) Saccharomycotina (which includes the "true yeasts", including among others Saccharomyces cerevisiae [Bakers' Yeast] and Candida albicans, the most frequently encountered fungal pathogen of humans and often the agent responsible for vaginal yeast infections and thrush and some toenail infections, among others human medical woes);
Ascomycetes occur in terrestrial, marine, and freshwater habitats and many species play a major ecological role as decomposers. They range from microscopic to the size of large "mushrooms". The key characteristic of the Ascomycetes is the production of ascospores (by meiosis, usually followed by mitosis) in sac-like asci (singular: ascus) as part of their sexual reproduction (ascomycetes also reproduce asexually via the production of conidia, which are formed by mitosis at the tips of haploid conidiophores). In many ascomycete clades, these asci are enclosed in "fruiting bodies" (ascocarps), which include some very familiar forms such as truffles and morels.
In the vast majority of the symbiotic associations known as lichens, the fungal partner is an ascomycete (and a large fraction of ascomycete species are known from lichens). Many mycorrhizae (mutualistic associations between fungi and plant root systems) involve ascomycetes as well. A number of agriculturally important plant pathogens are ascomycetes--but so are the fungi that gave us penicillin and many cheeses (Ropars et al. 2012) and several ascomycete species have been major model organisms for research in genetics and cell biology.
(James et al. 2006; Hibbett et al. 2007; Kirk et al. 2008)