Bike frames today are generally made of one of the following materials:
Hi Tensile Steel
Aluminium alloys of various grades
Steel frames are the cheapest type of bike frame because to achieve the equivalent strength of other materials, the frame will be heavier.
High tensile steel (Hi-Ten) is actually a mixture of Iron and Carbon (therefore technically it is an alloy).
Chromoly Steel is a steel alloy that contains Iron, Chromium, Carbon and some other bits and pieces. It is lighter than high tensile steel for the same strength properties.
Reynolds Steel is actually a series of different grades of steel alloys each given a number which indicates the make up of the steel tube and the treatments that it has undergone in manufacture. For example Reynolds 725 is a heat treated chromoly steel (the heat treatment means thinner walls can be used in the steel tubes achieving the same strength properties). You can find out more about the different grades of Reynolds Steel by visiting their website at reynoldstechnology.biz
Aluminium is an alloy which has a higher strength to weight ratio than the steel alloys. It is not as stiff as steel and for this reason to achieve the required strength and stiffness in a bike frame, aluminium tubes tend to be of a larger diameter. In fact, a good quality steel alloy frame can be just as light as an aluminium frame. As with other alloys, the aluminium is made up by combining different elements and therefore there are different grades of aluminium alloy:
6061 aluminium alloy contains aluminium and magnesium this is perhaps the most common family of alloys found in bicycle frame manufacture.
7005 aluminium alloy contains aluminium and zinc. Untreated 7005 aluminium is strong enough to use to make bike frames and if untreated, the cost of manufacture and therefore the cost of the bike is minimised. However, to achieve the equivalent strength as 6061 aluminium you need to use more of it so the bike will end up being heavier. To create a lighter bicycle frame, heat treated 7005 alloy can be used. The heat treatment means overall less material is needed to achieve the required strength and stiffness. the downside of this treatment process is the cost it adds to the frame manufacture and therefore to the bike.
Some bike manufacturers apply their own custom manufacturing processes to their bike frames and then sell these under a brand name. These are in effect aluminium alloy frames that are "tweaked" by the manufacturer to alter their properties.
Carbon Fibre Frames
Carbon fibre is easily the most expensive of the bike frame materials and is most commonly found on high end road bikes. Carbon is lighter and stiffer than either aluminium or steel and a bike frame can be made in one piece with no joins or with joins as with aluminium and steel frames.
The necessary strength and stiffness of tube can be achieved with different profile tubing - therefore allowing for more aerodynamic tubing.