As of patch 2.1, all potions and elixirs that added a buff were designated as either Battle Elixirs or Guardian Elixirs. The Battle Elixirs being those that increase attack power, strength, damage versus certain monsters, etc. Elixirs will stack with ability-generated buffs (Trueshot Aura for example) but will not stack with other Battle Elixirs.
Players can only have one Guardian Elixir and one Battle Elixir active on them at one time, or one Flask, which counts as both Elixir slots.
Review the Elixir article for a full list of items classified in-game as Battle Elixirs.