The Washington Post has a quick guide to who's being investigated for what:
Russian election meddling and possible collusion with the Trump campaign
This is where it all started. James B. Comey, who led the law enforcement investigation until he was fired as FBI director May 9, testified last week before the Senate Intelligence Committee that he has no doubt that Russia attempted to influence the presidential race by hacking the Democratic National Committee and launching cyberattacks on state election systems, among other tactics.
Possible attempts to obstruct justice
Comey testified last week that while he was still head of the FBI, he told Trump on three occasions that the agency was not investigating him, individually. “Officials say that changed shortly after Comey’s firing,” The Post reported Wednesday.
Possible financial crimes
We know less about this prong than the other two. The Post reported last month that “in addition to possible coordination between the Kremlin and the Trump campaign to influence the 2016 presidential election, investigators are also looking broadly into possible financial crimes — but the people familiar with the matter, who were not authorized to speak publicly, did not specify who or what was being examined.”
Meanwhile, Attorney General Jeff Sessions personally asked congress to prosecute medical-marijuana clinics, so that we can spend millions of Federal law-enforcement dollars hurting sick people. Gotta love the Republican Party.