The Sentinel is a Variant Class for the Druid in Dungeons & Dragons 4th Edition. Part of the Essentials subline, it was introduced in "Heroes of the Forgotten Kingdoms" as a "simplified" take on the Druid.
The Sentinel's flavor does not really elaborate on why they are different to normal druids, but in practice, they are very different; Sentinels focus on the magical side of the druidic arts, forsaking the shapeshifting powers of the standard druid for spellcasting prowess and the aid of a loyal animal companion, sort of a primal wizard with a beefier familiar. To better enforce the use of these companions, all Sentinels gain Combined Attack, a multiple-uses-per-Encounter attack that replaces use of any other Encounter power which allows you and your pet to attack at the same time.
Sentinels choose a season to draw power from at first level, with Heroes of the Forgotten Kingdoms providing rules for Spring Sentinels and Summer Sentinels and Heroes of Elemental Chaos gave a third option in the Wastes Sentinels.
Spring Sentinels have a wolf companion, proficiency with the scimitar, a +1 bonus to attack rolls with scimitars, spears, daggers and sickles, and a +2 bonus to Heal checks.
Summer Sentinels get a bear companion, increased damage dice with maces and quarterstaves, and a +2 bonus to Athletic checks.
Sentinels of the Wastes gain a living zephyr companion (Essentially a little blob of wind), gain +1 AC and Reflex when using spears or staves, and a +2 to Endurance checks.
At 1st level, they can pick two choices from a list of classic druidic knacks. At 7th levl, they gain the ability to talk to plants and animals 3/day.
|Dungeons & Dragons 4th Edition Classes|
|Player's Handbook 1:||Cleric - Fighter - Paladin - Ranger - Rogue - Warlock - Warlord - Wizard|
|Player's Handbook 2:||Avenger - Barbarian - Bard - Druid - Invoker - Shaman - Sorcerer - Warden|
|Player's Handbook 3:||Ardent - Battlemind - Monk - Psion - Runepriest - Seeker|
|Heroes of X:||Blackguard* - Binder* - Cavalier* - Elementalist* - Hexblade* - Hunter*|
Mage* - Knight* - Protector* - Scout* - Sentinel*
Skald* - Slayer* - Sha'ir* - Thief* - Vampire* - Warpriest* - Witch*
|Settings Book:||Artificer - Bladesinger* - Swordmage|
|Others:||Paragon Path - Epic Destiny|
*·: Non-AEDU variant classes
Same name but completely different. An archetype that any class can take to become the master of fighting in armour. Imagine Wizard in fullplate. You can sleep in armour, and could destroy it to stop lethal damage.