That is a rather vague question.
Potions can be either drunk, thrown at an enemy, mixed with other materials, or be synthesized.
Even potions with useless effects can be mixed with some other material to make あぶない薬(Dangerous Potion), which can be mixed with the majority of other potions.
For the sake of simplicity, I will only list the ones with positive effects when drunk.
Purple Potions
緑茶(Green tea) : HP +20
玉露(Refined green tea) : HP +100
どくだみ茶(Houstonia tea) : maxHP+5
解毒薬(Antidote) : Restores power to max
ちからの薬(Power potion): +1 power, +1 maxpower
しあわせになれるくすり(Potion of happiness) : +1 Lv.
怪力薬 : Gives 怪力 status for a few turns.
無敵薬(Potion of invincibility) : Gives 無敵 status for 30 turns.
目薬(Eyedrop) : Able to see traps and Nitori
加速薬(Haste Potion) : Gives 加速 status for 20 turns
P缶(P can) : +25 P
Yellow Potions
秘蔵酒「酒呑童子」: High chance of critical hit for 20 turns
紅魔茶「スカーレットデビル」 : +HPregen for a few turns, +3 Power on this floor
乱神薬「三歩必殺」: After three turns, if Reimu hits an enemy, unleashes a 1HKO move on the whole room
透過薬「ステルスドリンク」: Turns invisible for a few turns.
弩級薬「超マッシュルーム」: Turns giant, double HP for a few turns,
緑茶オン・ザ・ロック : Heal all status and HP
緑汁 : +10 HP, heal status
良薬 : +100 HP, restores Power
活性丹 : +50HP, +1 Power, +1 maxPower
天眼薬 : Able to see traps, Nitori, and enemies, items, and the staircase, also able to see darker parts
漢方「冬虫夏草」: +10 maxHP, +1 Power, +1 maxPower, +5 maxBelly
日輪酒「花鳥風月」: Buffs attack for a few turns
良缶 : +100 HP, +25 P