Difference is only in terms of Clock Speed ,however both have same L2 cache memory.
Intel Core 2 Duo E6600 -2.4GHz , 4MB L2 Cache
Intel Core 2 Duo E6700 -2.66GHz, 4 MB L2 cache
Than you may ask "what is clock speed":
Every computer contains an internal clock that regulates the rate at which instructions are executed and synchronizes all the various computer components.The faster the clock, the more instructions the CPU can execute per second.
Well let me clear what is cache memory also:
cache is to speed up memory accesses by storing recently used chunks of memory.