UDIMM, or unbuffered DIMM, is a type of computer memory that does not have a buffer. This makes it less expensive than other types of DIMMs, such as Rambus DRAM (RDRAM) and registered DIMM (RDIMM). UDIMMs are used in commodity servers, workstations, and desktop computers. They are not compatible with each other and must be installed in pairs.
DIMMs are available in various speeds ranging from 66 MHz to 400 MHz. The most common being 133 MHz for DDR SDRAM DIMMs. There are also 200-pin SO-DIMMs which are commonly used in notebook computers. These operate at much lower voltages than standard DIMMs (1.8 V or 2.5 V instead of 3.3 V) to conserve battery power.
While UDIMM provides some advantages in terms of cost savings, there are also some disadvantages to consider. One is that UDIMM is not as widely supported as other types of DIMMs. This means that if you have a motherboard that supports UDIMM, you may not be able to find replacement modules as easily as you could for other types of Dimm's . Additionally, since UDimm's don't have buffers, they can be more susceptible to data corruption caused by electrical noise on the bus