When flying an ILS (Instrument landing system) you will be tracking a localizer and a glideslope. The localizer is a ground based navigation beam designed to line you up with the center of the runway. the glideslope (also a ground based navigation beam) is a vertical beam (usually 3 degrees) designed to provide a stable and safe vertical descent path to the runway.
An ILS has ground based vertical guidance, it is called a precision approach.
When flying the ILS to runway 29 in Teheran as you come closer to the runway you will reach an altitude where you must see the runway before you are allowed to continue to land. That altitude where you have to make the decision whether you continue to land; or discontinue the approach, is often referred to as "minimums". The minimums for a precision approach are called DA(H) Decision Altitude or Decision Height. The DA(H) is an altitude read on the altimeter where the pilot must make a decision: do I have the required runway in sight and can I safely land. If the answer is "yes", then at the DA(H) the pilot continues the flight and lands. If the answer is "NO", then the pilot discontinues the approach and flies to a safe altitude (Missed approach).
A precision approach has a DA(H) associated with it. In many cases the DA(H) is 200' above the runway touchdown zone.
An instrument approach to a runway without a vertical glide path is called a non-precision approach. Examples would include: a localizer only approach or a VOR approach (there are others as well).
When flying a non-precision approach; as you get closer to the runway you are allowed to descend to a lower altitude with the intent of being able to visually see the runway so as to land. But with a non-precision approach there is no vertical guidance provided by a ground based navigation aide.
While flying a non-precison approach the lowest altitude where the pilot may descend without first seeing the runway of intended landing is called the MDA (minimum descent altitude).
With a precision approach minimums are DA(H)
With a non-precision approach minimums are the MDA.
As with most things in aviation there is usually "more" to the subject. For example CAT II & Approaches, minimums based on radar altimiters, PAR approaches (nothing ground based) and LNAV-VNAV based RNP-RNAV approaches....so there is always "more to the story". I have tried to be simple yet correct in my explanation. The above is from a U.S. pilot with the acknowledgement that in other parts of the world differences will occur.
Khodafas Farid.
Respectfully,
Northbeach