Cross hair directors show pitch/roll (attitude) commands. These commands are followed by positioning the att.indicator's fixed aircraft symbol (which inidcates present aircraft attitude over the artif. horizon) over them (mechanically, it is the other way around, of course). The autopilot follows the commands precisely or the manual pilot with a tiny delay. It is very clear and very precise. The display is sort of classical ILS presentation - up/down, left/right.
Airbus does provide the pilot with Flight Path Vector indication ("bird") and associated Flight Path Director ("moustache"). This indication is pilot selectable.
The bird appears as a "winged circle" less than 1 cm in size, the moustache as a barette with triangles at sides, hence the term. When displayed ontop of each other, the moustaches forms what looks like feathers to the little wings of FPV.
The Bird provides aircraft reference display by means of real velocity vector (trajectory) in PFD. In level flight, for instance, the attitude reference is 3 deg up, the bird's wings would be placed exactly on the horizon line (level flight) and the whole thing could be shifted to a side - indicating a drift. Wind from left -> bird on the right side (you're being blown to the right).
The moustache to bird is just as the flight director is to aircraft reference symbol on attitude indicator.
As you select FPV/D on, director bars are removed from view and aircraft reference symbol is dimmed somewhat. Your reference is now flight path, not aircraft attitude. But also, your FCU (MCP) guidance changes from V/S to degrees pitch and form heading to track.
This is a great way to fly non-precision approaches. With autopilot on, at FAF you select appropriate descent angle and final approach track. The flight guidance computer will provide guidance via FPD - the moustache to command the trajectory chosen. The autopilot will follow and aircraft will remain on profile and track regardless of speed, thrust, configuration and wind changes.
(rated prooners: Trying to keep it simple, I know when you deselect FDs you keep bird and AP stays on and you still can steer using FCU).
Now, comes the tricky part. Imagine it is not the autopilot that follows the "moustache" it is you, using trajectory as flying reference. It is not what humans are built for. We survive flying planes by using attitude as a reference. Airbus aircraft are flown like any other aircraft. Not only is this true, but it is also "the airbus golden rule #1" (basically A-N-C expanded to six items).
Today, Airbus say that for manual flight, FPD (moustache) is not used. The symbols are to small to follow, the indication si too confusing. You should switch the director part off (just like you could with normal FDs) and use the bird only which is very comfortable.
For dynamic manouvres like a go-around, the bird is of little use because it shows real trajectory with great precision when you need pitch/power settings.
I believe that Gnadenburg says that on some early manufactured models if crew had FPV/D for approach, during go-around it remained the reference system in PFD. Which, I agree, is totally incomprehensible and in IMC potentially deadly. Latter models have modification that automatically reselect classical FD bars for go around.
Imagine G/A with FPD/V
FD
Edited for VERY silly typos and some stupid grammar.