Raid Buffing

Every class and each spec now possess raid wide buffs and debuffs since Wrath of the Lich King. There is a lot of overlap in this with other classes which can bring the same buffs/debuffs as the Death Knight. As such, it is important to coordinate with the other classes and of course the other Death Knights in the raid as some of these require talents being spent and certain spells being added to your spell rotation.

Here is the run down on what each type of Death Knight offers, there is some overlap between specs:

Blood Death Knights

Horn of Winter provides a raid wide strength and agility buff. This buff can be performed by all Death Knights, regardless of spec. An equivelant buff can also be provided by Enhancement Shamans that spec for improving their totems.
Icy Touch debuffs the enemy causing them to attack 20% slower. This can be done by all Death Knights, but only Frost Death Knights will achieve the full 20%, the other specs will only have a 14% reduction debuff. Other classes able to apply an equivalent debuff are Feral Druids, Warriors and Protection Paladins
Abomination's Might is a Blood Death Knight only buff that gives all raid members a 10% increase to their attack power. A similar buff can also be performed by Trueshot Aura from Markmanship Hunters and Unleashed Rage from Enhancement Shamans.

Frost Death Knights

Horn of Winter provides a raid wide strength and agility buff. This buff can be performed by all Death Knights, regardless of spec. An equivelant buff can also be provided by Enhancement Shamans that spec for improving their totems.
Icy Touch debuffs the enemy causing them to attack 20% slower. This can be done by all Death Knights, but only Frost Death Knights will achieve the full 20%, the other specs will only have a 14% reduction debuff. Other classes able to apply an equivalent debuff are Feral Druids, Warriors and Protection Paladins Improved Icy Talons is a raid wide buff that increase the attack speed of raid members by 20%. The other class able to do this buff is Enhancement Shamans that spec to improve their Windfury Totems.

Unholy Death Knights

Horn of Winter provides a raid wide strength and agility buff. This buff can be performed by all Death Knights, regardless of spec. An equivelant buff can also be provided by Enhancement Shamans that spec for improving their totems.
Icy Touch debuffs the enemy causing them to attack 20% slower. This can be done by all Death Knights, but only Frost Death Knights will achieve the full 20%, the other specs will only have a 14% reduction debuff. Other classes able to apply an equivalent debuff are Feral Druids, Warriors and Protection Paladins Ebon Plaguebringer is a debuff applied to the boss that can only be done by Unholy Death Knights. It increases magical damage taken by 10%. Other classes with similiar debuffs are Balance Druids and Affliction Warlocks