By opening treasure chests, defeating enemies and bribing monsters, you can acquire spheres for use on the Sphere Grid. There are five types of spheres.
The most common type, often won from common enemies. Power Spheres enable you to increase a character’s HP, Strength, and Defense. Mana Spheres increase MP, Magic, and Magic Defense. Speed Spheres increase Agility, Evasion, and Accuracy. Ability Spheres teach the character new skills or special abilities. Fortune Spheres enable the character to activate Luck nodes, but they are the hardest red spheres to come by.
The rarest type of spheres, these enable one character to acquire abilities already acquired by another character. Characters can learn special abilities, skills, white magic, and black magic spells that have already been learned by others.
These could be considered the most important spheres to acquire, because they remove locks that block progress on the Sphere Grid. The appropriate level key sphere is required to open a particular lock. The higher the level, the harder the key sphere is to come by. All of the characters will eventually run into Lv. 3 and Lv. 4 Locks in their area of the Sphere Grid.
These spheres enable you to turn an empty node into an attribute increase of some sort. The increase is higher than the typical node attribute node. For example, if you use an MP Sphere to fill an empty node, the node will increase a character’s MP by 40 points rather then the usual 20. You should consider using the MP and HP spheres on Magic users like Yuna and Lulu to compensate for their general lack of HP nodes on their grids and make spell casting easier. Consider using the Luck Spheres on Rikku, so that her Steal and Mug abilities are improved. Also, try to use them whenever two or more characters are near each other, so that more than one character can immediately benefit from the new node.
These enable you to move a character’s marker to another point on the Sphere Grid, even if the character does not have any Sphere Levels to expend. A Return Sphere lets the character return to any node that they have not previously activated. A Friend Sphere lets a character move to any other character’s location on the grid, and a Teleport Sphere lets a character move to any node activated by any other character.