Power weapon

Cato Sicarius using a Power Sword to fuck over the Necrons.

Power weapons are a type of weapon in Warhammer 40,000. Power weapons look just like normal melee weapons, but what makes them actually useful is that they are sheathed with an energy field surrounding the blade. This energy field can in turn slice through even the toughest of armor.

Types of Power weapons

Crozius Arcanum

The signature weapon of Chaplains, the Crozius Arcanum is a mace encased in energy that serves as both a symbol of office and a weapon.

Power Sword

In a nutshell, the power sword is an example of power weapon technology applied to a sword. It's easily the most common power weapon (Seriously, fuck what Warhammer 40,000: Space Marine says about this, anyone with a functioning set of eyes and access to GW's own site can see it's bullshit), and is used not only by the Space Marines, but by the Commissariat, Inquisition, and Imperial Guard, who, likewise, often issue it to Sergeants and Officers. Most of the Primarchs who had swords used Power Swords.

Shock Mace

An older-model power weapon favored by the traitor legions, the Shock Mace is functionally a Crozius Arcanum without the shield emitter.

Power Axe

More readily available in the armoury than Power Swords, and far more popular amongst the Loyalist Space Marines, Power Axes are single or double-bladed axes with their edges sheathed in energy. One would think that having technology available to them would better allow them to produce other types of power weapons, but the Adeptus Mechanicus are idiots like that. Power Axes are especially favored by the Space Wolves and some Chaos Lords.

Power Fist

One of the most iconic weapons of 40k, Power fists are massive gauntlets which are encased within energy when activated. They are intended to be worn with Terminator armour, but frequently you find people without power armour wielding power fists. The God-Emperor of Mankind had a power fist on a left hand, which he used to famously put Leman Russ in his place. Russ later said that the subsequent headache was from the drinking contest they had.

Orks, in their looting tendencies, have taken to mimicking power fists as power klaws, which are like power fists except that they usually have 2-4 digits, and end in talon-like shears instead of articulated fingers.

Chaos Power Fists usually have longer, claw-like fingers that allow the user to slash with it, as well as crush.

Lightning Claw

Beakie Raven Guard love Lightning Claws.

Lightning Claws are a trio of razor-sharp claws affixed to a gauntlet, each blade of which is a small power weapon. They are normally used in pairs. Favored by Terminators and Chaos Raptors, Lightning Claws are a bit unwieldy, but are much more likely to land a damaging hit on a target when used by a trained user (in tabletop, this translates to rerolling failed to-wound rolls). The Raven Guard in particular often employ Lightning Claws.

Power Lance

A weapon used almost exclusively by the White Scars, Power Lances are energy sheathed lances which the White Scars use in charging their opponents on their bikes. You can imagine the results.

Thunder Hammer

Big fucking hammers that smash anything they touch. Unlike other power weapons, Thunder Hammer's only activate their energy fields upon contact with a target. Come in one-handed and two-handed varieties, but both functionally are the same weapon stat-wise.

Daemonhammer

It's a big thunder hammer that is consecrated with sacred oils and unguents to allow it to defeat Daemons.