Life
Life is an integer or thing use to determine if a certain unit is living or not. Life is commonly used in some tooltips as HP or Health Points. Life can be changed (automatically) with the Strength Attribute. This Strength Attribute is an essential thing for life which modifies how many life you gain per point of this attribute and how many should be regenerated per second.
Life is also shown as bars above every unit. This maybe green (for full HP Units), yellow or somewhat orange (for 40-50% HP units) or red (units that have very low HP). You could show this bars by clicking the "ALT" key on your keyboard or defining it at the Options. For the options, click Gameplay then check the box beside "Automatically Show Health Bars" (only works for Patch 1.23 and above).