Here are is an explanation of the Jet and Copter HUDs (via Reddit and Reddit)
F\AF/A 18 Super Hornet: See above full sized.
Mil MI-28 Havoc: See above full sized.
As an additional note, aligning the I-beam with the vertical bit of the crosshair ensures that your rockets will fly to their target. This also works by aligning the 'O' in between the two | | in the Sideslip Indicator (see image above).