Potion
A potions primary use is to help reload your deck faster for attack cards, without increasing your reload counter. You have a supply of these when you first begin with the game. While it does reload your deck, it does not however bring back cards that you used in a sleight. The only way to retrieve those cards would be to end the battle. Potion cannot be used in any sleights.

"Quickly reloads attack cards with no charge time required. It does not work on cards that cannot be reloaded."

Hi-Potion
A Hi-Potion has the same exact purpose as a regular potion does, except that it actually brings back any cards that are lost through a sleight. You can obtain one of these by taking a visit to Olympus Coliseum. This allows you to use many sleights in a battle and be able to bring them back without doing serious damage to your deck. But any cards that are lost due to a Zantetsuken can't be brought back by this.

"Quickly reloads attack cards with no charge time required. It does not work on cards that cannot be reloaded."

Mega-Potion
Mega-Potions have the same abilities as it's predacessors, but here's what makes it superior. When you use it, it resets your reload counter back down to one. After you defeat Riku III you obtain this card. Very useful for a long heated battle. However it still does not restore cards lost to a Zantetsuken.

"Quickly reloads attack cards with no charge time and resets the reload counter. Even normally unreloadable cards are restored."

Ether
Think of an ether is a potion, only it reloads your magic cards. You can obtain an ether while in Agrabah. So when you use an ether it reloads your magic cards without increasing your reload counter. It does not reload cards lost in a sleight or by a Zantetsuken.

"Quickly reloads magic cards with no charge time required. It does not work on cards that cannot be reloaded."

Mega-Ether
A Mega-Ether rivals a Mega-Potion. Both have the same effects but the ethers deal with magic. Defeat Vexen I to get this card into your deck. The Mega-Ether restores all magic cards in your deck, doesn't increase your reload counter, resets your reload counter and restores cards lost in sleights. As usual, don't expect it to bring back cards lost in a Zantetsuken.

"Quickly reloads magic cards with no charge time and resets the reload counter. Even normally unreloadable cards are restored."

Elixer
If you have the cash, an Elixer might be just the thing to give your deck a little extra something. You first obtain this card in the Hundred Acre Woods. What an Elixer does is reload all attack and magic cards without increasing the reload counter. It even reloads cards lost in sleights. And yes, no restoring Zantetsuken lost cards.

"Quickly reloads attack cards and magic cards, including normally unreloadable cards."

Mega-Elixer
Truly an item that surpasses all of them. Most expensive card in the game, both it's worth it. Obtainable at Destiny Islands. Returns all cards when used, including ones lost in sleights. By now you know that no Zantetsuken lost cards are going to be returned. This card also resets your reload counter back to one. It can also be combined with other cards to make Holy.

"Quickly reloads attack cards and magic cards, including normally unreloadable cards. The reload counter is also reset."