Short answers, pulled from the story.
The Strategic Arms Limitation Talks began in Helsinki in November 1969. Subsequent negotiating sessions alternated between Helsinki and Vienna, lasting until the SALT I agreement was signed in May 1972.
SALT I, signed on the 26th of May 1972, froze strategic ballistic missile launchers at existing levels and capped the combined NATO and United States SLBM-capable submarine fleet at 50 vessels with a maximum of 800 SLBM launchers. It also limited each country to one ABM deployment site.
The US Senate never ratified SALT II because President Carter withdrew the treaty from consideration in January 1980, following the Soviet invasion of Afghanistan in late 1979. Although neither side ratified the treaty, both honored its terms until 1986.
Leonid Brezhnev and President Jimmy Carter signed the SALT II treaty on the 18th of June 1979 at a ceremony held in the Redoutensaal of the Hofburg Palace in Vienna.
MIRVs, or multiple independently targetable reentry vehicles, were missile warheads that carried several nuclear warheads and decoys aimed at different targets. They made existing anti-ballistic missile defenses difficult and expensive to operate, which complicated arms limitation discussions throughout the SALT talks.
SALT II was superseded by START I, a 1991 agreement between the United States and the Soviet Union that proposed limits on multiple-warhead capacities. A successor, New START, was eventually ratified in February 2011.