All you really need to know about serial comms is that, as far as the Arduino is concerned, data are transmitted and received as bytes; it is up to you to recombine those bytes as larger data types, or break down the data types into bytes.
The other thing is, serial is relatively slow - a single byte at 9600 baud takes nearly a millisecond to transmit, so just because "Serial.available" has returned a 1, don't assume that as soon as you've read that character that there'll be another immediately available.
You could go round "loop()" many times in the time it takes for another character to be ready, even if they were transmitted one after the other.