Allright, I guess it's technical time. There are several types of cartridges, with MM(moving magnet), MC(moving coil) leading the pack. Some others are MI(moving iron, or variable reluctance) which is most popularly used by Grado, strain gage(used by Stax),Electret(popularized by Micro Acoustics) and some Ceramic types used on the old 60's cheap record players. I can't think of any other types right now. Out of these, the first 3, MM, MC, MI, are primarily what you will encounter, unless you are specifically searching out the other types.
Although there are technical differences, MM and MI are usually lumped together. The MM has a small magnet mounted to the cantilever, and it moves between 2 coils in the cartridge body to generate the signal. The MI has a piece of iron mounted to the cantilever, and it moves between a set of coils and a magnet mounted in the cartridge body to generate the signal. They are typically categorized together because they have similar output characteristics. The generally accepted standard output characteristic for MM type cartridges is at least 2.5mv into a 47k ohm load in the phono section. Many are much higher, going all the way up to 6 or 7 mv. The average is about 4.5mv. These will require typically 35db-40db of gain in the phono section for useful listening levels. This is general info, and not intended to be specifically targeted at any one application.
The next one is MC cartridges. These use 2 very small coils of wire attached to the cantilever, and it moves between 2 or more magnets to generate the signal. These may have very different output characteristics in terms of voltage and load, depending on brand and model. The 2 main groups are High-Output MC and Low-Output MC.
The high-output MC will usually have 2mv-2.5mv of output. This is so they can be used in MM phono sections, without too much gain mismatch. They are a little on the low gain side, but most MM phono sections will handle them. They also are typically loaded into the 47k ohm standard MM-type loading.
The low-output MC has much smaller coils on the cantilever, and generates much lower voltage output. They may range from .1mv - 2mv. Generally they will be below 1mv. These require much higher gain in the phono stage, on the order of 45db-75db, depending on the output of the particular cartridge. They also require different loadings on the impedance scale, ranging from as low as 6 ohms, to has high as 47k ohms, with the typical around 100-1000 ohms. This is directly related to the brand and model of cartridge. In some cases a step-up transformer is used to bridge the gap in the gain needed for the phono stage, so a low output MC could be plugged into a MM phono stage, with the use of the step-up transformer, and the loading done on the transformer end.
Why would anyone want to use a low-output MC, when it has such difficult needs? The answer is that the low-output MC has a very light coil structure and it responds faster to the miniscule signals coming off the record. These are the fastest and most detailed types of cartridges. The high-output MC attempts to retain as much speed as possible, but has a larger heavier coil structure to increase the output. So the high-output MC is typically not as fast and detailed as a low-output MC, but it is easier to use in a standard MM type phono section. The MM cartridges use the heaviest system, which has a magnet on the cantilever instead of coils.
In all fairness, there are many very good MM type cartridges available today, and they can be as good or better than some MC types which may not be as well engineered,as Marakanetz mentions above. But in the high-performance area, a well designed and built low-output MC cartridge can have the very best sound of all.