Most definitions of these terms are circular. A sales lead is defined as a potential customer, which tells you nothing about when a record becomes one or what changes when it does.
Each guide here gives the definition, then the part that matters operationally: what the term means for your data, your process and the number you report at the end of the month.
They are also the entry points to the deeper guides. If you are new to outbound, start here and follow the links into the deliverability and data clusters.