I'm going to make a wild stab in the dark, that you are probably moving bitcoins from an exchange, because if you are new to this you are probably not making multi-party payments.
The earlier of the two transactions (green) is showing several previous addresses, paying several others. This would occur for instance when you withdraw from an exchange: ie they will be paying many people who are all making withdrawals, from coins that were deposited at many addresses.
The second payment, I am going to assume is a consolidation payment. As you say you only made one transaction, I can only think that this is either an exchange consolidating your deposit (because your first transaction was from one exchange to another), or else you withdrew to an online wallet service which is doing the same thing - consolidating. (Beware)
There is a third alternative, and that is that your machine has been compromised and you have just lost some bitcoins. I hope not.