Some wallets for Bitcoin and Ethereum will give you a list of about 15 words to write down instead of giving you a private key.
From what I understand, the list of words is derived from the key, and can be used to recreate it.
The question I have, is basically whether or not ALL (or virtually all) implementations of this system are compatible. Imagine I try to recover my wallet using the seed years later on a totally different wallet. Can I be confident it will work. Are all seed mnemonics using the exact same math, and will they likely continue with the same scheme (or a backwards compatible one) decades from now?