Normal cloak

"In the Star Trek universe, the cloaking device supposedly generates a "spatial distortion" which causes light and sensor rays to travel around the ship, so that nothing is reflected from its surface. Such an in intense distortion – equivalent to an enormous mass concentration, such as that in the vicinity of a black hole or neutron star – would certainly crush the ship and crew unless additional measures were taken to counteract the effect inside the distortion."

However, that theory is thrown right throught window by existence of graviton-based shields - hence my idea that cloaking device just modifies shields in order to hide ship. That also explains why Scimitar was able to raise shields while cloaked - it had primary and secondary shields, and only secondary were used for cloaking. Primary shields remained up.

Romulan cloaking device
 
(unfinished)
 
Klingon cloaking device

 
(unfinished)
 
Federation Interphase cloak
 
See here.