Terminology

Yaoi: Put simplistically, yaoi means something depicting a romantic or sexual relationship between two males. The term is used loosely by most Western m/m fans, but yaoi actually only refers to m/m themed drawings done by fans of other people's characters. This is similar to the term doujinshi, which is applied to the amateur Japanese comics that are about an existing series. Many yaoi artists start out in the doujinshi industry. Yaoi is an acronym for "yama nashi, ochi nashi, imi nashi', which means "no moutain (i.e. no climax), no point, and no meaning". From this definition one can gather that yaoi generally focuses on sexual themes and not so much on the relationship or emotional aspects.
June: Pronounced "ju-ne" (not like the month of June). This is the correct term to use for a serious m/m story, be it professional or amateur, and usually refers to original character creations. Long-standing m/m stories with a solid base are considered june. Quickie fics and PWP's are not.
Shounen-ai: Literally, "boys' love" (shounen=boy, ai=love). Shounen-ai denotes stories that focus not so much on sex, but on the emotions and love between two adolescent boys. The act of sex doesn't even have to occur at all.
Shota-con: Short for "Shota Complex"; derived from the name "Shotaro", which is supposedly a typical name for little boys. In a general sense, shota-con stories center around the relationship between either a grown man and a prepubescent boy or two prepubescent boys.
Seme: The term used for the "top" in the m/m relationship. The canon seme is always taller than his partner, stronger, and generally more masculine.
Uke: The seme's counterpart-- his "bottom" partner. The classic uke has a slender, less masculine build, and is often portrayed as more emotionally sensitive than his seme.