Here is "impeachment" in a nutshell:

Will Trump be impeached?  If the GOP keeps the Senate and no criminal complaints are proven,  there is no chance for a successful impeachment trial. 

First, impeachment is not, by itself, the removal of a president. It is a single step in a multi-step process: The House Judiciary Committee must recommend impeachment. Then the House of Representatives carries out an impeachment vote. If that vote succeeds (by a simple majority), a trial takes place in the Senate (presided over by the chief justice of the Supreme Court), where a two-thirds majority is then required to convict.

{Because the GOP controls both houses of congress,  impeachment really is not possible until the third year of Trump's first term  ~  editor]

Second, what are the grounds for impeachment? Article II, Section 4 of the Constitution reads, "The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors."
Has Trump committed treason? The Constitution has a very specific definition of treason: "levying War against [the USA], or in adhering to their Enemies, giving them Aid and Comfort." Trump hasn't waged war on the United States, and because we aren't at war with Russia, giving them state secrets probably isn't treason either. Bribery? Not that we know about. High crimes and misdemeanors? This is a very definite "maybe."

[In other words,  there is no evidence for any criminal behavior, treason or bribery  ~  editor]  
 
"High Crimes and Misdemeanors" was a catchall phrase, common in 18th century British law. It didn't refer to a special kind of crime, but rather regular crimes committed by the high and mighty – people with political power. The framers of the Constitution made it clear that, although they didn't want presidents to be fired every time Congress got antsy, they did want to leave an escape hatch in case the chief resident of the White House went bad.

What exactly qualifies as a "high crime and misdemeanor"? Strict constitutional constructionists argue that it must be something actually illegal, a crime for which someone could be convicted in a court of law. Broad constructionists, instead, believe that impeachable offenses are anything Congress thinks is unacceptable. Or,  to quote then Rep. Gerald Ford during the failed 1970 attempt to impeach Supreme Court Justice William Douglas, "an impeachable offense is whatever a majority of the House of Representatives considers it to be at a given moment in history."
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Related Rolling Stone article:   http://www.rollingstone.com/politics/features/how-impeaching-trump-would-work-w482643