Republicans and Democrats caucus differently.
After some organizational business and speeches, Republicans write down their pick for president and turn in their secret ballot.
Democrats break into groups according to their preference, plain for all to see. If a candidate fails to achieve the viability threshold of at least 15%, he or she is eliminated and supporters fall in behind another Democratic candidate. Or they can go home, though that is considered bad caucus form.
Why do the parties do things differently?
Because they can. Caucuses are party meetings, independent of any government or election agency. So the parties set their own rules. Democrats were deeply divided over the Vietnam War, so they fashioned the viability rule as a way of forging consensus and avoiding a split of the party into multiple small factions.
Why does Iowa go first?
Because for a long time, nobody much cared. The caucuses have been taking place, in some form, since the early 1800s. The presidential preference vote was an add-on, a bit of fun for insiders who were gathered to attend to party business. In 1972, the national media picked up the results of the informal balloting and that helped George S. McGovern, a surprising favorite, push past better-known rivals to win the partys nomination. Four years later, underdog Jimmy Carter used a strong Iowa showing to win the White House, and Iowa has occupied a prime spot on the political calendar ever since.
http://www.latimes.com/nation/politics/la-na-iowa-caucuses-explained-20160131-htmlstory.html