Slardar is a melee strength carry and initiator.
His Slithereen Crush active and Bash passive gives him good lockdown, whilst his ultimate, Amplify Damage, supplies sufficient armour reduction on enemies that he is able to slice his way through all but the tankiest heroes even without an enormous amount of items.
Skills
Sprint is skilled at level 2 or 4 but is maxed last. Use cautiously; you take extra damage when using it.
Slithereen Crush is skilled and maxed first.
Bash is skilled at level 2 or 4 and maxed second.
Amplify Damage is always skilled (levels 6, 11 and 16). Make sure to cast on your target before right-clicking them.
Items
Starting Items: A set of Tangos, a Healing Salve, a Stout Shield, 2x Iron Branches.
Early Game: Power Treads, Magic Wand.
Core Items: Armlet of Mordiggian, Blink Dagger.
Luxury Items: Black King Bar, Assault Cuirass, Crimson Guard, Heart of Tarrasque, Desolator, Daedalus.
Situational Items: Vanguard, Drums of Endurance, Vladmir's Offering, Heavens Halberd.