The line between decentralised and centralised wallets is blurred. Electrum lets you keep your private key but connects to the Bitcoin network through one of a limited number of servers. Blockchain.info lets you keep your private key but you must access the Bitcoin network through its website. Coinbase is fully centralised because you don't know your private key, but Electrum and Blockchain.info are partly centralised.
How is electrum centralised when your private keys never leave your pc and you can do what ever you like with your coins. The only thing electrum needs is an internet connection. Centralized mean some central supreme authority watching over you and making decisions for you, decentralized mean no central authority, you are free to do what you like, thats the big difference between Electrum and Coinbase which don't let you know your private keys.