RAM for most modern computers are available as DIMMs (dual-inline memory modules). Notebook computers use SODIMMs (SO stands for small outline).
There are different types of DIMMs, but most DIMMs for personal computers are DDR2 (double data rate 2) memory. This means that each memory bus clock cycle can have two transfers. This is a trick to boost the number of memory content transfers while given the physical constraints of printed board layout.
Memory is rated by several attributes. First, there is the maximum frequency at which content can be transferred. This ranges from 400MHz (400 million cycles per second) to 1066MHz (1066 millions cycles per second). The naming of memory is somewhat confusing. First, there is the memory bus standard, which is DDR-X, where X is the frequency of memory transfers. However, the name of memory modules is in the form of PC2-Y, where Y is the number of bytes that can be transferred per second. Y is eight times X because there are 8 bytes for each memory transfer.
There is no need to use memory that is faster than that specified by the Northbridge chip of a motherboard. Using memory modules that are slower than the maximum speed of a Northbridge degrades systems performance, but it does not cause any harm.