Accurate info posted about what a tweeter is. I just want to add one thing. The lower the frequency, the slower (as already mentioned), and less directional it is. The higher, the faster, and more directional it is. So you'll never see a tweeter vibrate unless the resistors are letting low frequencies through and it's straining to reproduce the long waves.
Now to the important part about speaker placement. You can here bass sounds no matter where the speaker is placed in the vehicle. Remember the 80's when you could hear bass thumping cars from inside your house? The high frequencies go in one direction - like a ray gun. So, if your tweeter is faced away from your head, or is down by your feet, you won't hear them clearly. Placing the tweeter higher, and even angling it toward you head, gets the sound to you without having to drive the volume up. High frequencies are also more reflective (they act like a pool ball on a pool table) where bass tend to just seep through things. Remember the good old sedan cars that had speakers on the back shelf pointed toward the sky? The passengers could still hear the symbols crashing because the sound reflected off the rear window and were redirected toward them.
'01 XJ/ 4.5 Short arm, AA SYE, JKS Qkr discos, JKS SuperNerfs, D30,C8.25, 4.56, rear ARB, Logan's front bumper, XRC8, 33" KM2s