Any one network connection – eg at the LAN layer – has a data rate: the rate at which bits are transmitted. In some LANs (eg Wi-Fi) the data rate can vary with time. Throughput refers to the overall effective transmission rate, taking into account things like transmission overhead, protocol inefficiencies and perhaps even competing traffic. It is generally measured at a higher network layer than the data rate.
The term bandwidth can be used to refer to either of these, though we here use it mostly as a synonym for data rate. The term comes from radio transmission, where the width of the frequency band available is proportional, all else being equal, to the data rate that can be achieved.
In discussions about TCP, the term goodput is sometimes used to refer to what might also be called “application-layer throughput”: the amount of usable data delivered to the receiving application. Specifically, retransmitted data is counted only once when calculating goodput but might be counted twice under some interpretations of “throughput”.
Data rates are generally measured in kilobits per second (kbps) or megabits per second (Mbps); the use of the lower-case “b” here denotes bits. In the context of data rates, a kilobit is 103 bits (not 210) and a megabit is 106 bits. Somewhat inconsistently, we follow the tradition of using kB and MB to denote data volumes of 210 and 220 bytes respectively, with the upper-case B denoting bytes. The newer abbreviations KiBand MiB would be more precise, but the consequences of confusion are modest.