The treaty reinforced the Romulan Neutral Zone and stated the grounds that would result in the violation by either side which could be considered an act of war. (TNG episode: "The Defector") One of the stipulatons of the treaty was an express prohibition of the development of cloaking technology by Starfleet.
A secret amendment was made to the treaty in 2371 which allowed limited use of cloaking devices by the Federation as long as it was supervised by a Romulan military officer. This allowed the USS Defiant (NX-74205) (I) to be loaned a cloaking device for its operations against the Dominion. However, the amendment stated that only a single Defiant class vessel was allowed the use of the cloaking device and only in the Gamma Quadrant (DS9 episode: "The Search, Part I")
In 2358, Admiral Erik Pressman attempted to circumvent the Treaty of Algeron by attempting to create an Interphase cloaking device which was secretly developed by Starfleet Intelligence which was tested on the USS Pegasus. (TNG episode: The Defector)
In 2371, the USS Defiant was deployed at Deep Space 9 for operations against the Dominion and the use of the cloaking device was overseen by Subcommander T'Rul of the Romulan Star Empire. (DS9 episode: The Search, Part I)