Glenn_CostaRica
1. What is the difference between a SPV (simplified payment verification) and a full node?
A full node is an operating station that keeps an updated full version of the blockchain, that serves as a security point that verifies constantly that rules are being respected by miners and by all users of the network, and that also serve as a first instance of attention for new transactions that need to wait in the mempool before being mined. An SPV, on the contrary, is a kind of mini-node that shows some of the behaviours and functions of a node, but in a very limited way. Basically, an SPV is a node-like system that does not store the whole blockchain and cannot provide all the services of a full node, but that is directly connected to a full node and, therefore, can provide services indirectly. Usually, smart phone Bitcoin wallets are SPVs, for example. The level of security and reliability of an SPV will depend on many factors like who is the developer of the SPV and on the full node it depends on.
2. What does it mean when a transaction is broadcasted?
It means that the package of data that the transaction consists of, is sent to other nodes, usually to as many nodes as possible.
3. How does a miner pick which transactions that gets added to the next block?
First, the miner will look for transactions that its own node and other nodes keep in the mempool. All transactions that are waiting to be mined are in the mempool. If the amount of transactions in the mempool does not exceed the capacity of one block, then, probably the miner will take all the transactions he can and mine them. If the number of transactions in the mempool exceeds the maximum capacity of a block, then, the miner will usually take the liberty of choosing those transactions that offer the highest incentives, or higher fees.