Latency vs. Bandwidth
latency is the time it takes to send a minimal (0 byte) message from point A to point B. Commonly expressed as microseconds.
bandwidth is the amount of data that can be communicated per unit of time. Commonly expressed as megabytes/sec or gigabytes/sec.
Sending many small messages can cause latency to dominate communication overheads. Often it is more efficient to package small messages into a larger message, thus increasing the effective communications bandwidth.