Composition
Like Sterling Silver, Pewter is an alloy.
Pewter is comprised of tin, with the remainder consisting of copper, antimony, bismuth and in lower grade pewter, lead.
Pewter varies in it's composition. Examples would include the composition of 94% tin, 1% copper, and 5% antimony or perhaps as softer composion of 97.5% tin, 1% copper, and 1.5% antimony.

History
Pewter is believed to have been first used during the Early Bronze Age (3300–2200 BC) in what is now the modern day countries of Turkey, Syria, Lebanon, Jordan, Cyprus, Israel and the Palestinian territories as well as Iraq.

Alloys
Put simply, an alloy is a mix of two or more metals. In the case of Pewter, we have four solid state metals combining to form the alloy.
Alloying metals with one another often enhances their properties. With Pewter, alloying tin with copper, antimony and bismuth enhances its strength.

Further Reading
To read a little more about Pewter, take a look at the detailed description from Wikipedia, the free encyclopedia.
Melting Points of Metals and Alloys