A fundamental element of vote theory, a voting block is simply any group of voters who intentionally vote the same way.
A voting block can consist of as few as 1 member (some theorists also include the null set) or as many as the entire constituency being offered the vote. Usually they tend to consist of either a large group of minority voters, or a plurality voter plus enough minority voters to achieve a majority.
A Simple Example
The Widget Company is owned in equal parts by Alice, Bob, and Charles. In order for any business to proceed, it must receive a majority vote - that is, at least 2 votes. In the simplest system, each of the co-owners forms their own voting block having 33 1/3% of the votes of the company.
A quick review of power distribution: the power of any voter is their ability to "make or break" a vote. To determine this, first you must determine all possible voting sets:
A, B, C
And then determine how many times a particular voter, when removed from a set, changes the result of the election (these are called critical votes.) In this case, only 3 of the vote sets are critical - AB, AC, and BC. If you remove any one voter from any of these sets, the election changes from a "yes" to a "no." Since each voter appears twice in these sets, the power distribution for this company is (A: 2 (33.3%); B: 2 (33.3%; C: 2 (33.3%)).
But what if Alice and Bob are husband and wife?
In this case, as a whole, their interests in the company are mutual and self-serving. Thus, they form a voting block comprising 66 2/3% - a majority block. Now the only vote sets are:
And since the only critical vote set on here is the "AB" set, the power distribution is (AB: 1 (100%)). Sorry, Charlie!
Voting blocks are much more often seen in business than in politics, as the stakes and decisionmaking are less ambiguous as a whole. Many companies which offer shares also offer systems to vote by proxy, a form of block voting in which a shareholder allows another shareholder (often a much larger one) to vote for them, generally based more on a common philosophy than any specific viewpoint.
While voting blocks make great headlines in election years, they are generally overrated in terms of getting out the vote and influencing elections. Single issue voters rarely number larger than 5 or 6% in any election year. However, in smaller elections, voting blocks have played a large part in the American political process - almost all of the modern party conventions and nominating procedures have been dominated by block voting, compromising, and occasionally, betrayal of the block.