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|