The first author is usually the person who has made the most significant intellectual contribution to the work, in terms designing the study, acquiring and analyzing data from experiments, and writing the manuscript.
Most of the time, the first author also presents the study during a conference. The corresponding author is often a senior researcher who helps the first author. He/she is responsible for the correspondence during the process of submission. Being the first author is highly valuable.
1st Author: Person who did most of the work, like write the manuscript, and do most if not all of the experiment. 2nd Author: Person who helped out the most, and/or person who mentored the 1st author (e.g.: if 1st author was grad student) the most.
the lead author