A testnet is the replica of a bitcoin’s code and its network. The coins from testnet are of no value and are used to test applications.
A target is a number that is a representation of the present difficulty. In order to have a valid block, the hash of its header must be less than the target.
Two parties can make payments to each other without having to directly exchange bitcoin addresses as there could be an interception. Instead, a key is used to negotiate and this is called a stealth payment.
Some wallets come as SPV wallets and what they do is rather than validate the entire blockchain, they rely on nodes to calculate the balance. Be careful because the nodes could give inaccurate statements and lie about the validity of transactions to the SPV wallet.
SPV stands for simplified payment verification and it is meant to allow nodes alert each about invalid blocks in the network without having to verify the whole block. SPV has b=not yet been implemented and may not even be possible.
Every bitcoin transaction has an input that leads to the source and also at least one output. If an output acts as a source for another transaction, it is considered spent. The sum of unspent outputs whose private keys you own gives you your bitcoin balance.