The US Mint produces two basic types of coins, the Uncirculated, which we call a "business strike" to signify that it was produced for business or commercial purposes (spending money) and the Proof, which is an official, legal tender, limited edition collector's item.

 

The proof is produced by hand selecting and inspecting a planchet (blank coin) and then highly polishing it.  They then prepare obverse and reverse dyes, which are also carefully inspected, cleaned, and highly polished.  The blank coins are hand fed into the coinage press one at a time and are struck twice by the dies to bring out all the design details in high relief.  This is a slow speed process in which extra pressure is used.

 

In contract to business strikes, proofs are struck in very small numbers. These then are the finest quality coins produced by the Mint.  This doesn't necessarily mean that a proof is automatically rarer than a circulated coin because rarity is determined by how many coins have survived in a given grade, not by how many were originally struck.

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