Q What is a special counsel?
A A special counsel is appointed by the US Attorney-General - or, in this case, Deputy Attorney- General Rod Rosenstein because Attorney-General Jeff Sessions is recused from the Russia case - to oversee a particular criminal investigation. Such an appointment is made when the Justice Department has a conflict of interest or when it is otherwise in the "public interest" to do so.
Q What does such a special counsel do?
A The special counsel oversees investigators and ultimately decides whether or not criminal charges are warranted. If charges are warranted, the special counsel can prosecute the case.
Q Does this mean US President Donald Trump or his associates have committed a crime?
A Not necessarily. Mr Robert Mueller, appointed as special counsel, could bring charges, but he could also decline to do so.
Q What can the special counsel investigate?
A In this case, Mr Mueller was specifically tasked with investigating "any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump".
Q How wide an investigation is he allowed to conduct?
A The special counsel was also given the authority to investigate "any matters that arose or may directly arise from that investigation". That would seem to include possible efforts to impede the investigation, including an alleged request by Mr Trump to shut down a probe related to former national security adviser Michael Flynn. The law also allows the special counsel to investigate efforts to impede his own work.
Q Can the special counsel demand that Mr Trump release his tax returns?
A While Mr Mueller has the power to subpoena any documents, including tax returns, he would have to explain why they are relevant to his investigation.
Q To whom does the special counsel answer?
A The special counsel is not subject to the "day-to-day supervision" of any Justice Department employee, but the buck does not necessarily stop with him.
The Attorney-General - or in this case his deputy, Mr Rosenstein - can "request that the special counsel provide an explanation for any investigative or prosecutorial step, and may after review conclude that the action is so inappropriate or unwarranted under established departmental practices that it should not be pursued", according to the law on the topic.
Q How much weight do a special counsel's findings carry?
A The Attorney-General must "give great weight" to the views of the special counsel, and if he intervenes, must notify the chairman and the ranking members of the congressional judiciary committees with an explanation for his actions.
Q Can a president be impeached for something he did before taking office - as in this case which involves events that allegedly occurred while Mr Trump was campaigning last year?
A Law professor Michael Gerhardt, who works at the University of North Carolina and has written a book on the impeachment process, said: "The short answer is yes.
"If it is determined as fact, then it would appear what he did wrong can be related to his being elected, and that creates a nexus."
• Additional information from The Washington Post