I am completely new to everything bitcoin related. I am using Electrum 2.2.
To receive bitcoins from a poker site, I created an address using the Bitcoin Core wallet.
When I realized that synchronizing takes forever, I downloaded Electrum and used the address from Bitcoin Core
to create my first Electrum wallet.
...
In order to spend bitcoins from an address, you need the private key. Only your Bitcoin Core wallet has the private key to the address you are using. You can acquire the private key from Bitcoin Core and import it into your Electrum wallet.
Start Bitcoin Core, go to the debug console, and use the "dumpprivkey" command to print out the private key. Then go to Electrum and import that private key. I don't know the details, but you can probably figure them out.
this guy is right, just having a wallet, you can see the balance as well as transactions, but you only have access to the funds and are allowed to spend them if you have the private key. its a great security, enough that i remember some guy put his encrypted wallet file up for download somewhere and challenged people to take the ~8
BTC stored on it; it was possible to see the balance as you had the wallet, but impossible to send the bitcoins to your personal address as the wallet was encrypted and you didnt have the private key.