Thank you both for your answers. Here is more ;-)
So I just need a seed from Electrum? Will creating a seed also create btc address?
This seed which is a few words only, saved on a piece of paper, is enough to have my BTC secure whatever happens?
What if Electrum disappears? Would I be able to use the seed (list of few words) with another program or with an online service like bitgo or greenaddress to get them back? Or how would I get them back?
And where are the private keys in this case?
And where are the BTC actually?
And what can I do with the USB device? The USB device will store the private keys (how many are there?), but not the seed? Or both?
Thank you :-)
Electrum will create a new seed for you if you do not have one. This seed is used to generate many BTC addresses (As many as you need, there is no limit). All of the keys for the bitcoin addresses it generates come from this seed. And that is correct, as long as you have that seed stored somewhere, you can use it to re-generate all the Bitcoin addresses you've used in your wallet to re-create it and have control over your money. If Electrum disappears it would still be possible to use that seed and restore your wallet.
The private keys in this case are all derived from the seed, in what's called a Hierarchical Deterministic wallet (HD wallet). Those keys themselves are not usually stored, just the seed used to create them, because as long as you have the same seed you can re-create all of the same addresses. Bitcoins are never really "in" any computer or device, in the sense that if that device is broken/lost the Bitcoins are destroyed. The Bitcoins reside in the network, and the keys that are used to spend them are on the devices. If the device that has the only copy of the key to your Bitcoins is destroyed, those Bitcoins are lost in the sense that nobody can spend them. But if you have your seed written down somewhere else, you can use it to re-create your keys and regain access to those Bitcoins.
The USB devices (hardware wallets) use the same method as Electrum in that they use a seed to create addresses. As long as you have the seed stored somewhere safe, even if you lose the hardware wallet or it stops working, you still have access to your coins.
Bitcoin Addresses look like this: 1KsFhYKLs8qb1GHqrPxHoywNQpet2CtP9t. These are the things that you give to other people so they can send you money. Each bitcoin address is created from one private key, and each private key creates only one bitcoin address. The seeds used in modern wallets can create any number of private keys, so you can create any number of Bitcoin addresses.