Personal shields are a ground item, which provides additional protection. It acts like a health bar that absorbs damage before your character's health takes damage, but can be bypassed by certain attacks. Notably hand to hand attacks (such as from a Klingon Bat'leth which all Klingon characters begin with) seem able to bypass the protection offered by personal shields.
Varieties: Their basic power can be measured by their rank, starting at MK I, every two ranks higher requires a higher officer grade (ie, MK III requires Lt. Commander)
Rarity: They have the basic Common, Uncommon, Rare, and Very-Rare varieties. Additional abilities include resistances to certain types of damage (ie disruptor), shield point cap, and/or increased recharge rate.
Brikar Bite: Due to the high recharge rate they probably benefit the most from either cap for general use or resistances when you know your going to be fighting alot of one type of enemy. Also engineers can "heal" personal shields with the Quick Fix ability.
Personal shields can be equipped to both your character as well as Bridge Officers.