In simple terms, the number of possible combinations of bits that can make up a key of any given length can be expressed as 2 raised to the n, where n is the length of the key. Thus, a formula with a 40-bit key length would be 240, or 1,099,511,627,776 possible different keys.
In general, keys with more bits offer stronger encryption than keys with fewer bits.
No matter how many bits a key contains, it can always be broken, given enough time and computing power. The key length chosen should be directly proportional to the data being protected: the more confidential the data, the higher the number of bits in the key should be.
Working against us is the speed of modern computers. Although the number of possible keys is indeed large, specialized computers can now try that many combinations of keys in less than a day. Ron Rivest has explained cryptographic strength determined on the basis of key length, given the current state of computing power.