Does that mean I store the coins in the "system" rather than on my hd?
Can I get them back even after not being online for 30 years or so (Assuming I'm still alive by then..)?
That's right, they are stored in the blockchain. Bitcoins does not actually exist anywhere, the blockchain is just a bunch of transactions, starting with a generation transaction with just a receiver and no sender. Your wallet file only contain the private keys needed to sign a transaction from the corresponding bitcoin address.
I suggest reading the Bitcoin white paper by Satoshi Nakamoto, the wiki and, if you'll understand it, the source code. Links at bitcoin.org.