Pottery is any object made of fired (baked) clay. (Clay is a mixture of water and fine-grained earth.) Pottery can be divided into three main types according to the temperature at which it is fired.
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Pottery fired at low temperatures, known as earthenware, retains the appearance of baked clay. Earthenware is porous; consequently, it will slowly leak water.1
At higher temperatures, clay begins to vitrify. Vitrification, the conversion of an ordinary solid into glass, is achieved by heating a material to a sufficiently high temperature, then allowing it to quickly cool. While the particles of an ordinary solid are neatly ordered, the particles of a glass are haphazardly arranged, causing glass to be brittle and see-through. Pottery that has vitrified to a significant extent is called ceramic. (Firing clay at an extremely high temperature results in pure glass, which is not classified as ceramic.)
Ceramic (sometimes called "china") has a much harder surface than earthenware, and is non-porous. Low-temperature ceramic, termed stoneware, typically has an opaque, stone-like surface. High-temperature ceramic, referred to as porcelain, typically has a somewhat translucent surface.2 It can be difficult to distinguish between stoneware and porcelain, however.
A work of pottery is often finished with a liquid coating, of which the two main types are slip and glaze.
Slip is simply watered-down clay, with a consistency like cream.2 Slip may be applied to hide joints (e.g. where a handle is attached to a jar) or to change a vessel's surface colour (e.g. a white surface over a brown jar).
Glaze is a mixture of water and powdered ceramic or glass.2 When baked, a coating of glaze will fuse into a ceramic veneer (if powdered ceramic was used) or glass veneer (if powdered glass was used), even at the low temperatures at which earthenware is fired. Glazing thus allows an earthenware vessel to be given a smooth, hard, non-porous surface.
While glass-based glazes are transparent, ceramic-based glazes are opaque. Only the former type is typically applied to ceramics; both are applied to earthenware. Opaque glaze is also known as enamel. One type of enamel is tin-glazing (in which the ceramic powder is produced by heating tin oxide), which bakes into a white glaze.2 Enamelled earthenware can be difficult to distinguish from ceramic.
2 - "Pottery", Encyclopedia Britannica. Accessed November 2009.