Measurement and Signals Intelligence deals specifically with missile and anti-missile systems, warhead design/testing and capabilities of various systems relating to the above. Not to be confused with SIGINT (Signals Intelligence) which is a completely separate technical collection effort. MASINT mainly concerns itself with what a potential adversary is developing in terms of nuclear weapons technology and the capabilities of those systems. The information is typically used in the estimation of nuclear weapons program capability and the design of anti-ballistic missile (ABM) defense systems.

Currently the United States government (specifically the DSP or Defense Support Program and SDS or Satellite Data System) employs satellites with the codename of HERITAGE for the majority of it's MASINT collection efforts. Almost all MASINT collection efforts fall under STRATCOM (Strategic Command,) control due to their direct relationship with nuclear weapons technology.

Additional Pieces of MASINT:
Unintentional Radiation Intelligence (RINT): Particles and radioactive footprint of specific weapons or the boosters that carry those weapons. Can also include emissions from missiles or warheads during launch, flight or terminal phase operations.
Optical Intelligence (OPINT): Appearance before, during or after launch cycle, possible damage assessments, data on support equipment around or present at a launch or detonation.
Infrared Intelligence (IRINT): Infrared signature of weapon or booster during launch, flight or detonation.
Nuclear Intelligence (NUCINT): Similar to RINT, with considerably more emphasis placed on warhead designs and resulting particle release from a specific weapon or family of weapons. Nominal yields, possible design defects and damage estimates are derived from this information.
Laser Intelligence (LASINT): Use of lasers for targeting/designation, guidance or defense.

Public sources:
The U.S. Intelligence Community, Third Edition. Jeffery T. Richelson 1995, pgs. 233-234.