I understand the key management and security benefits of using subkeys. However, I'm not sure whether subkeys are partial keys with limitations or full-fledged keys (just as the master key). Both the master key and the subkey look like a pair of numbers to me so that I'm wondering whether the word sub really comes from their usage but not their capability.
The difference is that you have a master key, which you probably don't want to revoke or let expire often, and subkeys, which you actually use for crypto.
The subkey can be revoked, expire etc, but it's also proof that you're the owner of the master key. In essence, people trust you (i.e. you master key), but you just use the subkey. So, when the life time of one of the subkeys ends, you just use a new one – and don't have to rebuild your web of trust.