It depends on the site. To clear up something though, there usually almost nothing in your actual internet cache besides cookies that can be used to hack. (never say never, though.)
Cookies on the other hand often contain your username and password. The reason you can't read them is that they are usually "hashed" or encrypted. Hashing some text creates a unique string of a certain length that's made up of numbers and letters. Hashes cannot be undone or decrypted. A hacker could figure it out eventually, but it takes some time. The more complex your password is, the longer it takes to crack a hash. A simple 4 letter password could literally take less than a minute to solve. A more complex 12 letter password with letters, numbers, and symbols could take days or longer.
However.... cookies can ONLY be read by one of two methods. Viewing them from your own computer (as you did) or by the originating site itself. The latter means that a myspace.com cookie can only be read by myspace.com, not yahoo.com or any other site. If the site has a vulnerability, hackers can use the site to read your cookies, a vulnerability which myspace.com has had in the past.
I don't actually know how most hackers hack accounts, but I would presume that it is most often done through 3 methods: phishing, direct social engineering, and brute force. None of those 3 methods actually require the hacker to interpret or decode your cookies or cache.
Hope that helps.