RedHead is correct.
Impeachment is an investigatory process by the entire body of Congress. Congress has the power of subpoena, but I'm not sure if they can enforce it against Administration members covered under self-declared executive privilege. That issue would be adjudicated by the USSG one item at a time. Without the power of subpoena and prosecution to convince lower order players to testify, and testify truthfully, the impeachment would be useless. Obama could also simply extend executive privilege to every person that Congress called, and it'd be the law of Omertà versus democracy.
However, the impeachment can be referred to the Senate at any time for deliberation and confirmation, at which time the President would be considered "Impeached" and removed from office.
The process is so unbelievably difficult, regardless of cause and evidence, that the GOP has simply given up any pretense at oversight. The USA truly has it's first imperial President, totally untouchable. We all know that.
Most importantly, Obama figured that out long before he ran for President. He knew the limitations of Congressional oversight over the office, and pressed it to maximum advantage. Ultimately, unless the electorate pushes it's representatives and Senators to impeach, there's no teeth in the process.
Until Obama, Presidents considered the decorum of the office, and strode carefully when they pushed the Constitutional boundaries of the office. Obama just never gave a damn. When you get a 60's (or in this case, post 60") communist radical in the White house, it's all over, Constitutionally speaking. Ditto, for any fascist, such as one who declares that military officers "will obey my orders - they will obey", in reference to illegal orders.
That is why I see Hillary and Obama as no greater threat than Trump. The republic is screwed either way, when there are Constitutional constraints on the office of the Presidency, and authoritarians take control.
We all blog and huff and shout and get all angry about this stuff. In reality, it's too late. The USA will never be what it once was short of a bloody revolution (e.g. 1860.V2), because authoritarians never surrender power. They take it, hold it, and never relent.
