The idea of foreign monitors observing elections actually has a fairly long history. The first election with international monitoring was in 1857 when French, British, Prussian, Russian, Austrian and Turkish representatives supervised a plebiscite in Moldavia and Wallachia.[3] Although there certainly were other cases in the interim, the post-World War II world provided more opportunity and the practice developed more quickly. Fast upon its creation, the United Nations was asked to monitor elections in Korea and Germany. As decolonization accelerated in the 1950-60s, so too did the UN's "first generation" involvement in founding elections to ensure they were free and fair. So-called "second generation" election monitoring missions are more comprehensive and have become more common with the end of the Cold War and the growing global consensus on the value of democracy. Beginning with Namibia's 1989 vote, election monitoring has often become part of a broader mandate of peacekeeping. Here, election observation is part of a multi-faceted international effort to support a peace agreement and help with the reconstruction of political systems and economies.

What is Electoral Monitoring ?
is the observing of an election by non-partisan, usually international observers. The main purpose of election monitoring is to ensure an election process is fair. Monitors can do little to prevent abuses, but they can ensure such abuses are made public. Criticisms by monitors can remove an election's legitimacy, but only if the observers are themselves considered unbiased. A notable figure is often appointed honorary leader of a monitoring organization in an effort to enhance its legitimacy. The first monitored election was that of a plebiscite in Moldavia and Wallachia that was monitored by most of the major European powers. Election monitoring was uncommon until after the Second World War.
http://en.wikipedia.org/wiki/Moldavia
The elections that are seen to be monitored are normally in countries where the democracy is seen as somewhat unstable and where there is a perceived threat that the election might be stolen. A team of monitors observed the 2004 United States election, after concerns of voter inaccuracy in the 2000 U.S. election.