The 'green coloured' materia found in Final Fantasy VII. Most of the time you can buy these at shops at various towns. Offensive spells usually add elemetal damage (such as fire, lightning). Also healing spells fit in this category, such as Restore (cure, regen, etc).

The "Purple coloured Materia" found in Final Fantasy VII. Usually equiped to boost a character's stats. For example HP Plus boosts HP and MP Plus boosts MP.