ColinEssex:
I will try to explain this in terms that you might understand because you are not familiar with USA government issues like this. I know it can be confusing because, heck, some folks here in the USA don't understand it either.
The issue with the President of the US is that he has certain protections and immunity to keep him from being pestered by trivial lawsuits and (dare I say it?) trumped-up charges of petty crimes. If everyone could charge him with some trivial crime then even if he was the nicest guy in the world, the party that is out of power would pester him relentlessly and keep him from the real work of running the country. Therefore, the President is immune to certain criminal charges.
However, the framers of the USA constitution decided that this was a bit TOO extreme, so they left in place a mechanism to oust a truly criminal president. Now here is where it might get a little legal-techie.
If you are an ordinary USA citizen and someone accuses you of a crime, there is some type of investigation by a police-powers agency, local or higher depending on the scope of the purported crime. The agency forwards the results of the investigation to the appropriate state or federal district attorney who can accept the charges or reject them. If the criminal charges are accepted, the DA convenes a grand jury to hear the details presented by the police agency. The grand jury only has to decide if there is a reasonable chance that a crime actually was committed. If so, they return an indictment and the accused person then will be given a court date for the criminal trial.
The grand jury DOES NOT DETERMINE GUILT. They only decide whether it is worth it to pursue the case. When the court goes into session, a petit jury is convened to hear the evidence by the prosecuting attorney (DA) and the defense attorney. The petit jury then decides guilt or innocence. At this point, if they voted guilty then the accused has been convicted of a crime.
Now, for the President... The USA constitution lets the House of Representatives bring charges and act as a grand jury, but the term used by the constitution is "impeach" rather than "indict." The Senate then acts as the petit jury and either convicts or exonerates the President.
In both cases, the grand jury (or the House of Representatives) CAN choose to return a "no true bill" result - meaning they do not recommend further consideration of the charges. IF they indict/impeach, then the petit jury/Senate can chose to acquit or convict. So what is really going on here is merely that there are a couple of oddball variations required as workarounds to offset the Presidential immunity from normal prosecution.
I hope this helps you understand the process, Col. I would tell you what it is like as compared to the UK system but I am not familiar with that.