"I just love scanning for lifeforms. Lifeforms... you tiny little lifeforms... you precious little lifeforms... where are you?" - Data
A lifeform is simply any living entity or organism. The study of lifeforms is known as biology and when dealing with alien life-forms can be further sub-categorized into xenobiology and xenology.
Starshipsensors and tricorders are primary instruments used in detecting or locating life-forms. This can be determined by identifying or localizing blood type, heart rate or metabolic setup.
There are incredible similarities between the lifeforms on different worlds in the Star Trek universe. A probable explanation for this may be the existence of the ancient humanoidsDNA during evolution.