Static magnets are composed of materials which are always magnetic. Household refrigerator magnets are. These types of magnets are typically used for small magnetic fields, but have a distinct advantage of usually being portable, small, and seldom requiring maintenance.
On the timescale of human experiments, an NMR researcher can view the Earth’s magnetic field as a static magnetic field. Although the Earths field is very low (approximately 0.0025 to 0.0065 Tesla), it is free, and present in places which are challenging to bring higher field magnets to.
Superconducting magnets are composed of superconducting materials in a closed loop circuit which can operate for long periods (years), so long as the superconducting properties of the material are maintained. These magnets have the advantage of reaching high fields, and maintaining highly stable magnetic fields. The superconducting materials typically need to be kept at cryogenic temperatures, and maintaining these temperatures typically requires cryogenic systems which can add substantial cost due to the cost of operating cryocoolers or frequent cryogenic liquid refills. The cost of these magnets also
Resistive magnets can reach very high magnetic fields, such as Bitter high field magnets used at several magnetic research facilities. These frequently need extensive cooling systems, often accomplished with expensive forced fluid cooling.
In common practice, solid state NMR experiments often use a combination of superconducting and resistive magnets to maintain and adjust magnetic fields which are homogeneous in the sample region.
Magnets are often discussed in terms of the proton resonance frequency. A “600” magnet is a 14.09 Tesla magnet where the proton resonance frequency is 600 MHz.