An "ordinary" flight plan, filed by a pilot whose goal is to get from point A to point B, has a fairly simple format: a point of origin, a route, and a destination. The origin and destination are typically airports. (Those of you who start or end your flights in places other than airports know what it's like to be not filing an "ordinary" flight plan). A route may be along airways, direct between points, or a combination of the two. They're fairly similar. On an ICAO flight plan, to go direct between points you can just write the identifiers of the two points side by side and it's assumed your route between them is direct. On a Canadian flight plan you can also put a D between them, typically written with a horizontal arrow across the middle of the letter. If part of the route will be on an airway, you specify the point at which you will enter the airway, and then name the airway, finishing with the point at which you will exit the airway. Specifying the points is the subject of this post.
If point A and B are airports, they are easy to specify, as every airport has a four-character code. I say character and not letter because while the bigger ones have four-letter codes, smaller airports may have digits included. Many airports I know the codes for by heart, while others I have to look up in the CFS. Other points along a flight plan might be NDBs, identified by two-letter codes, VORs, identified by three-letter codes, or intersections, identified by five-letter codes. And then of course there is me, going to random places that are not at any of these easily filed waypoints.
I can specify my location by giving a latitude and longitude, there's even an official format for such a waypoint: 5025N09823W, although I usually leave a space after the N. I hope that doesn't irritate anyone. Another way to give a location is "fifty nautical miles northwest of" some other point. You code it as the point, the bearing, and the distance. I once filed such a waypoint: EC330050, and the flight planner called me back to ask me what it meant. It was an obscure little NDB that no one ever filed to, so combined with the bearing and distance information, he just didn't clue into what it was.