There are two ways to obtain a Master Materia. At Bugenhagen's observatory in Cosmo Canyon, giving all mastered Summon Materia to the red Huge Materia will combine them into the Summon Materia. Alternatively, it can be obtained from the Kalm traveler by trading the Earth Harp (dropped by the Emerald Weapon superboss).
Master Summon provides the user with all summons. This is not possible without equipping the Master Summon, as a character cannot have enough Materia slots to equip all summons. Master Summon can be linked to a Support Materia to provide the benefits to any applicable summon. Though this can be useful with effects such as HP Absorb, MP Absorb, Steal as well, Added Cut, it is less useful with effects such as Final Attack and Sneak Attack, as they will use a random summon. Having two Master Summons and pairing one with Quadra Magic and one with Magic Counter, is the only way to summon Knights of Round four time in succession, as it usually doesn't work with Quadra Magic.
Master Summon comes with no HP penalties and saves a huge amount of Materia slots. It can thus be useful on any character, but it also provides no stat boosts. Characters with high Magic and MP stats, including Cloud, Red XIII, Yuffie, Cait Sith, and Vincent, will benefit from summons most. However, since it has no HP penalty, it also can be beneficial on characters more geared towards physical damage, such as Tifa and Cid, without taking many slots from them. The Materia is not useful when equipped on Barret, however, as Master Summon can have no AP, making it useless for his ultimate weapon Missing Score.