All damage in Warframe has a ‘Type’.
The physical types are:
Impact
Puncture
Slash
Weapons, abilities, etc. can and usually do inflict multiple types of damage.
For example:
Karak rifle (Impact: 21.1, Puncture: 8.1, Slash 6.8)
Paris bow (Impact: 9, Puncture: 144, Slash: 27)
Skana sword (Impact: 5.25, Puncture: 5.25, Slash: 24.5)
There are also elemental damage types that could be intrinsic to an attack or applied via mods.
Cold
Electricity
Heat
Toxin
Applying elemental damage mods to a weapon does not alter the existing damage types, it adds an additional type.
For example:
Karak rifle with max rank Cryo Rounds mod (Impact: 21.1, Puncture: 8.1, Slash 6.8, Cold: 32.4)
Got it? Good, because shit’s about to crazy.
You can combine multiple elements to create a combined element.
Example:
Electric + Toxin = Corrosive
Cold + Heat = Blast
Going back to our Karak example:
Karak + max Stormbringer (Impact: 21.1, Puncture: 8.1, Slash 6.8, Electric: 32.4)
Karak + max Infected Clip (Impact: 21.1, Puncture: 8.1, Slash 6.8, Toxin: 32.4)
Karak + Stormbringer + Infected Clip (Impact: 21.1, Puncture: 8.1, Slash 6.8, Corrosive: 64.8)
Let’s keep on going
Karak + Stormbringer + Infected Clip + Cryo Rounds (Impact: 21.1, Puncture: 8.1, Slash 6.8, Corrosive: 64.8, Cold: 32.4)
Note that combinations are formed in the order pairs are encountered from left to right.
Karak + Stormbringer + Infected Clip + Cryo Rounds + Hellfire (Impact: 21.1, Puncture: 8.1, Slash 6.8, Corrosive: 64.8, Blast: 64.8)
Mods that increase base damage, like Serration, will increase the damage of all elemental mods as well.
Karak + Serration (+100%) + Stormbringer + Infected Clip + Cryo Rounds + Hellfire (Impact: 42.2, Puncture: 16.2, Slash 13.6, Corrosive: 129.6, Blast: 129.6)
The combined elements are:
Cold + Electricity = Magnetic
Cold + Heat = Blast
Cold + Toxin = Viral
Electricity + Heat = Radiation
Electricity + Toxin = Corrosive
Heat + Toxin = Gas
Different defenses can be neutral, resistant, or vulnerable to different damage types.
Here’s a summary of what’s best to use versus the different enemy types:
Grineer: Puncture, Cold, Heat, Toxin, Corrosive, Radiation, Viral
Corpus: Impact, Cold, Electricity, Magnetic, Viral
Infested: Slash, Heat, Corrosive, Gas
Corrupted (Void) are special, because corrupted are made up of all the enemy types, so require specific combinations:
Impact + Viral
Puncture + Magnetic
Slash + Corrosive + Cold
None of this is necessary to play and have fun, but if you want to succeed at the highest level, you need to invoke the most powerful weapon in all of Warframe: knowledge.