The Druid's Shape Shifting skill tree
A Skill Tree is a kind of a categorized spellbook in Diablo II where you can see what skills are available to your character. It has three tabs at the right side. Clicking on the tab will take you to the corresponding skill tree.
The tree consists of rows of skills. The row indicates the level requirement to acquire the skill. The first (top) row of skills are available as soon as you create your character. The second row has a level requirement of 6, with each following row's level requirement increasing by 6, up to 30 for the bottom row.
Some skills have one or more arrows pointing to them. Any skill on the tree that points to another skill is a prerequisite. For example, in the image to the right, Fire Claws 
has arrows pointing to it from Feral Rage 
and Maul 
. This means that in order to get Fire Claws, you must already have at least one skill point in both Feral Rage and Maul.
Bow and Crossbow Skills — Magic Arrow • Fire Arrow • Cold Arrow • Multiple Shot • Exploding Arrow • Ice Arrow • Guided Arrow • Strafe • Immolation Arrow • Freezing Arrow
Passive and Magic Skills — Inner Sight • Critical Strike • Dodge • Slow Missiles • Avoid • Penetrate • Decoy • Evade • Valkyrie • Pierce
Javelin and Spear Skills — Jab • Power Strike • Poison Javelin • Impale • Lightning Bolt • Charged Strike • Plague Javelin • Fend • Lightning Strike • Lightning Fury
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Warcries — Howl • Find Potion • Taunt • Shout • Find Item • Battle Cry • Battle Orders • Grim Ward • War Cry • Battle Command
Combat Skills — Bash • Leap • Double Swing • Stun • Double Throw • Leap Attack • Concentrate • Frenzy • Whirlwind • Berserk
Combat Masteries — Blade Mastery • Axe Mastery • Mace Mastery • Polearm Mastery • Throwing Mastery • Spear Mastery • Increased Stamina • Iron Skin • Increased Speed • Natural Resistance
Elemental Skills — Firestorm • Molten Boulder • Arctic Blast • Fissure • Cyclone Armor • Twister • Volcano • Tornado • Hurricane • Armageddon
Shape Shifting Skills — Werewolf • Lycanthropy • Werebear • Maul • Feral Rage • Fire Claws • Rabies • Shock Wave • Hunger • Fury
Summoning Skills — Raven • Poison Creeper • Oak Sage • Summon Spirit Wolf • Carrion Vine • Heart of Wolverine • Summon Dire Wolf • Solar Creeper • Spirit of Barbs • Summon Grizzly
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Combat Skills — Sacrifice • Smite • Holy Bolt • Zeal • Charge • Vengeance • Blessed Hammer • Conversion • Holy Shield • Fist of the Heavens
Defensive Auras — Prayer • Resist Fire • Resist Cold • Resist Lightning • Defiance • Cleansing • Vigor • Meditation • Redemption • Salvation
Offensive Auras — Might • Holy Fire • Thorns • Blessed Aim • Concentration • Holy Freeze • Holy Shock • Sanctuary • Fanaticism • Conviction
Fire Spells — Fire Bolt • Warmth • Inferno • Blaze • Fire Ball • Fire Wall • Enchant • Meteor • Fire Mastery • Hydra
Lightning Spells — Charged Bolt • Telekinesis • Static Field • Lightning • Nova • Chain Lightning • Teleport • Thunder Storm • Energy Shield • Lightning Mastery
Cold Spells — Ice Bolt • Frozen Armor • Frost Nova • Ice Blast • Shiver Armor • Glacial Spike • Blizzard • Chilling Armor • Frozen Orb • Cold Mastery