For a exposition of this point, we'll base that tutorial upon the U.S. rules concerning the airspace. The U.S. reglementation is basically based unto the distinction made between the airspace lying over 18,000 ft, and the airspace lying below. The following tutorial may easily be extrapolated to the airspaces of other countries. Generally, as far as the VFR, and IFR rules, more detailed views are given in the tutorials dedicated to such subjects like the navigation under those conditions
A IFR (for 'Instruments Flight Rules') flight is a flight performed with the pilot flying with the plane's instruments only. The pilot, whatever the weather conditions may be, use only the gauges of the planes along with radio-navigation aids along his route! VFR pilots, at the contrary, have to pilot, on the one hand, function of visual clues, like such pitch of the plane will be measured from a visual clue between the horizon and a part of the plane, the panel mostly, and on the other hand, they have to fly their route through visual landmarks established on the ground
Any IFR flight assumes that the pilot be in contact with the aerial control authorities. Any IFR flight necessitates a flight plan!
One may fly IFR either below, either above the 18,000 ft mark. Below, it's about low altitude IFR, and beyond, high altitude IFR. Below 18,000 ft, a pilot will use the 'Victor Airways', those published IFR aerial routes, and, above, the 'Jet Airways', which are their high altitude counterpart. On the Victor Airways, altitudes have to be in plain thousands (3,000, 4,000 ft), as they are ordered to the pilot by the controllers, no matter the heading followed. Beyond 18,000 ft, the flight altitudes are chosen by the pilot and have to be plain thousands (21,000 ft, for example). Beginning at 28,000 ft, a flight altitude is called a 'flight level' and is abbreviated FL plus the altitude in hundreds of feet (like FL280 for an altitude of 28,000 ft)! To a flight, one can take like a basis -which is not specifically a US one- that a hemispheric rule is to be followed in both cases, be those in term of altitude, or of flight level. Flight between 0 and 179° are flewn with altitudes or FL the figure of the thousands is odd (3,000, 5,000, etc. or 250, 270, etc.) as flight between 180 and 359° altitudes or FL the figure of the thousands is even (4,000, 6,000, etc. or 240, 260, etc.)
As far as the aeronautical charts used by pilots are concerned, a IFR flight is either using low altitude IFR charts, or high altitude ones. Such charts are about of the same type, as they are mostly not taking in account the terrain below. They mostly are showing the aerial routes, the radio-navigations aids which define those, and the other aeronautical data of use. Such a low IFR chart is available with FS2002
As far numbers, data or rules useful to privale pilot flying IFR are concerned, here are some