The area in an L2 cache that identifies which data from main memory is currently stored in each cache line. The actual data is stored in a different part of the cache, called the data store. The values stored in the tag RAM determine whether a cache lookup results in a hit or a miss.
The size of the data store determines how much data the cache can hold at any one time. The size of the tag RAM determines what range of main memory can be cached. Many modern PCs, for example, are configured with a 256K L2 cache and tag RAM that is 8 bits wide. This is sufficient for caching up to 64 MB of main memory. If you add additional main memory, however, it won't be cached unless you also expand tag RAM. Some motherboards allow you to add additional tag RAM chips for this purpose but many do not.
For Pentium Pro and Pentium II microprocessors, the tag RAM is integrated in the chipset. Pentium Pros have an integrated L2 cache capable of caching up to 4 GB of main memory. Pentium IIs use a SEC daughtercard for L2 caches and can cache up to 512 MB.