Milling cutters are used on the milling machine to remove material from the work piece. Typically the milling cutter is revolving at a calculated speed (RPM) and work is fed to the revolving cutter at a calculated feed rate. Milling cutters exist in a variety of shapes to match the particular requirement of the job.
There are four basic types of materials used to make mill cutters.
High Speed Steel cutting tool (left), also called HSS, is a typical material for an end mill. It is inexpensive and exists in many sizes but has a limited cutting capacity. It will not cut very hard steels because, relatively speaking, it is too soft but is excellent for aluminum , mild steel, and other soft metals.
Cobalt high speed steel cutters (right) are improvement in hardness over the HSS
end mill. They are a little more brittle but will stay sharper longer due to the addition of Cobalt as an alloying element. Note that they look very similar in texture to the HSS cutters.
Solid carbide cutters (below) offer an increase in hardness and will allows the machining of many types of 
materials but are more costly than cobalt HSS cutters. It is very important to have a rigid setup and the appropriate speeds and feeds in order to maximize the use of the solid carbide cutter. Those cutters are less affected by heat at the cutting point than other types of cutters. They tend to be used for medium to large production runs. 
Coated cutters (right) have a solid carbide or HSS body with a coated cutting edge. The purpose of the coating is to make the cutting tool last longer or to be allowed to run the machine at a greater cutting speed. Titanium Nitrite coating is one of the most popular coatings.
End mills have different lengths. The shorter the length the more rigid in setup and the less vibrations on the machine. Short end mills (left) are called stub-length end mills
End mills are also commonly produced with double ends (right). Although double-end end mills are limited in size (rarely exceeding 1" in diameter), they cost less to purchase as compared to two single-end end mills.
Most end mills have spiral flutes with an angle of 30 degrees. That design is the most efficient (all of the above shown end mills have spiral flutes). Straight flutes (rendition below) are used rarely because they are less efficient. You may use them to make two sides of a cut that have to be parallel as in O.D. keyways on a shaft.
Note: Increasing the number of flutes from 2 to 3 or more improves your surface finish at equivalent feed and speed.