The transaction contains the recipient and the amount sent (among other fields). Before a transaction is mined into the blockchain the miner have to make sure the sender has enough balance to pay for the transaction.
To ensure this Ethereum manages a "world state" that contain the balance of each address, this state is secured trough the root world state stored in each block.
It is possible that you send several transaction but you do not have enough funds so they will remain in the "mempool". When you have enough funds again they can be processed immediately.
All data in Ethereum is encoded using RLP, transaction should be easily decoded. The Ethereum's Yellow paper (PDF) has the formal definition, but you should be able to find a library to decode RLP in your favorit language.