You are immune to a particular form of damage and suffer no WoundPoints or other harmful effects from such attacks no matter what. For each schtick you spend you become immune to an additional form of damage. Your choices and exceptions must be relevant to what you are and the campaign.
You are not only immune to damage; you actually cannot be directly affected by that force in any way. But you can still be indirectly affected by a creature that modifies its own abilities through a power you are immune to. Thus, a sorcerer could turn invisible, and you would not see him. A Creature may have Abysmal Spines modify its damage rating, but the damage itself is till from an Unarmed Attack.
Damage Immunity is much more appropriate to NPC monsters than to player characters; many plots revolve around finding the sole way of slaying some creature immune to normal weaponry. A Damage Immune PC can expect to become quite famous, and thus have his weaknesses known and exploited.
Here is a listing of common damage immunities. Note that many of these overlap. The GM may freely design other varieties. Some types of Damage Immunity may not be absolute, only reduce any Wound Points you take to one point.
Air
You are immune to all air-based effects, such as the Movement schtick of sorcery, lightning bolts and telekinetic attacks, falling damage and poison gas. See Air for examples of air powers.
Blast
You are immune to almost all effects of Blast. You are not immune to those blasts you yourself know; this costs one extra schtick. You can opt to be immune to only your own blasts for only one schtick pick. Note that some Blast schticks have special rules applicable to Damage Immunity, and these supersede this power.
Earth
Immune to damage from falling, being crushed, vehicle crashes etc. You cannot be grappled, held or entangled. You can walk on any surface, no matter if it could ordinarily carry your weight. This is also known as damage immunity to Accidents. See Earth for examples of earth powers.
Fire
Immune to explosions, fire, energy weapons etc. See Fire for examples of fire powers.
Fu
You are immune to all Fu powers and Races schticks. Characters using Fu powers to attack you simply fail.
Guns
Immune to all bullets, arrows and other missiles fired using Guns except one specific type or material. The typical limitation is silver bullets.
Influence
Immune to social and mental control and influence, such as Seduction, Intimidation, Influence, Domination, Suggestion, or Possession.
Life
You are immune to dangers harmful to living, organic creatures. This is typical of golems, undead and similar non-living creatures.
You cannot drown, starve or suffocate. You are not discomforted by normal extremes of temperature, but suffer normal damage from heat and cold attacks. You are also immune to Poison and disease, but not to acids. You never sleep. Your body is immune to SupernaturalHealing and to Polymorpism, but can be affected by Creation or Death magic, as appropriate.
Melee Weapons
Immune to hand-to-hand weapons except one specific type or material. This includes throwing and brawling weapons, but not grenades (you need Fire for that).
Powers
Immune to most Creatures and Supers Powers. You are not immune to those powers you yourself know; this costs one extra schtick. You can opt be immune to only your own Creature Powers for one schtick pick. Note that some Powers have special rules applicable to Damage Immunity, and these supersede this power.
Sorcery
Immune to all Sorcery schticks and the effects of magic items that imitate sorcery, except Blast and Summoning.
Unarmed Attacks
Immune to unarmed attacks, including Fu and Races schticks used unarmed. This does include close-combat Arcanotech, Creatures, or Supers schticks such as Abysmal Spines. Brawling weapons such as knuckles and Rippers are considered melee weapons.
You must select some form of unarmed attacks against which this power fails, such as Fu/Races or Creatures schticks and so forth. Or you can select one type of unarmed attacks - fists, bites, kicks, claws and soon. You cannot select something that is very rare or nonexistent in the campaign world
Water
Immune to all kinds of poisons, cold, stickiness, liquid attacks and similar effects, such as Slime, Acid and even drowning. You won't ever slip. You can attack freely underwater, but get no special water movement ability. See Water for more examples of water powers.