Whether you're an art student or big industry, molten metal has to be molded somehow. There are three basic ways to do this: sand molds, investment and permanent molds.
In a sand mold - perhaps the oldest molding method - sand bonded with moist clay is rammed in around a rigid pattern. The mold is seperated, the pattern removed, paths for the metal to enter are carved, and the mold is placed together again, ready for pouring.
Investment casting uses a setting material, plaster for instance, placed around a meltable or flammable pattern. After the plaster sets, the mold is burned out, removing the pattern. Since the pattern is lost in the process, this is often known as lost-wax casting (if wax is used for the pattern).
Permanent molds are a cross between the two, in that investment is usually a hard material, and sand molds are two (or more) assembled parts. The mold material is molded or carved; when ready, the mold halves are assembled and metal poured in. As long as there are no undercuts, the mold can be removed from the casting without damage to it, and can be reused.
The most common process, being cheap in small to medium quantities (say, up to a thousand). Very versatile, it can do any size from a nickel up to entire 100+ ton propeller s.
Most foundry sand is composed of 10 to 20% clay, 1 or 2% moisture, and the balance sand. Sometimes there are a few 'seasonings', such as charcoal for a reducing atmosphere inside the mold. This sand is known as 'green' sand, since it contains water. Special sands such as K-Bond and Petrobond are oil-bonded, the advantage being it requires less maintainance, such as keeping the moisture level exactly right...
The basic process is limited to patterns without undercuts, or at least where the total angles allow release. With cores, castings can get very complicated indeed. Finish generally isn't very good, but if you use very fine sand (or should I say silica flour) you can get pretty much perfect finish. Thickness is limited to about 1/8".
Popular for art casting, investment casting will capture just about every detail of your pattern.
A typical investment recipie is 2 parts sand to one part plaster, water until creamy. The investment is poured in around the pattern (held inside a flask), and when it sets it is burned out. The mold isn't very porous, so the gas inside can't escape too easily; however, the mold was just as hotduring the burnout as the metal coming in (at least with zinc or aluminum), so there isn't going to be any extra gas being produced as the metal comes in.
Finish can be very good, and with a bit of help (such as spinning the mold for centrifugal force, forcing the metal in with steam, or drawing it in with vacuum) can easily get nice and thin sections.
If you want ten thousand pieces with a bright finish and thin sections, this is the way to go. Typically, the mold is made from machined steel dies, hence the term die casting. Since the dies cool the shot very quickly, it's possible to have cycle times on the order of seconds!
Due to the cost and difficulty of making the dies, as well as the equipment to run them, this is basically limited exclusively to industry; even so, I've seen at least one design for the hobby caster.