Cloaking device
A cloaking device is a form of stealth technology that uses selective bending of light (and other forms of energy) to render a starship or other object completely invisible to the electromagnetic spectrum and most sensors. It has been encountered in varying forms over the centuries.
The purpose of cloak is to obscure visual identification. Romulan officers do not consider the cloak sufficient to bypass prepared defenses (TNG Face of the Enemy).
Vulnerabilities
In general, depending on their power sources, cloaked vessels are not shielded. Cloaking and decloaking generally creates a window of vulnerability, exploited by Riker in Generations to destroy a Bird of Prey.
The Scimitar in Nemesis was shielded during cloak. Worf was eventually able to target the Scimitar based on weapons fire and disable its cloak.
Cloaked vessels can be spotted by the naked eye such as in The Search for Spock, or the external visuals in Nemesis.