And how does is it different from other schools of Buddhism?
Mahayana Buddhism: The term "maha-yana" means basically "great vehicle" - this is a term that is used by this school itself. The term "Buddhism" is a bit of a modern neologism (new word), based on the founder, the Buddha. Before this new term, there was more usage of terms such as "Dharma", ie. teachings, to describe it.
The "great vehicle" holds that there are several possible goals of Buddhist practice. Of these, the highest goal is to attain the state of "supreme, right, full awakening" (anuttara samyak sambodhi). This differs from the basic awakening (bodhi) or liberation of other goals. These lesser goals are often referred to as the "sravaka-yana" (hearers' vehicle) and the "pratyekabuddha-yana" (solitary awakened one's vehicle). Together, these two vehicles are often called the "hinayana" (lesser vehicle / inferior vehicle).
Regards distinctions between the vehicles, there are a number of takes. However, the basic gist is to become an awakened one, ie. a Buddha, like Sakyamuni Buddha, or any one of a number of other Buddhas who have lived, do live, or will live, in various world systems across multitude universes. This goal is contrasted with the awakening of the other vehicles, corresponding to the disciples of Sakyamuni Buddha and these other Buddhas.
There are several descriptions of the path to this great vehicle goal of full awakening. However, the six perfections (sad-paramita), is probably the most common. The first five are charity (dana), morality (sila), patience (ksanti), energy (virya) and meditation (dhyana); together when combined with the sixth, knowledge / wisdom (prajna), they lead one to "cross over to the other shore" (param-ita) of "all knowledge" (sarvajnata). This process takes many thousands of millions of billions of eons. However, certain teachings claim to have methods that make all this possible in several, or even a single, lifetime.
The motivation for taking up the goal of the Mahayana, as opposed to the other two vehicles, is given as compassion. It is believed that only a fully awakened Buddha is fully capable of leading other living beings out of the continuous suffering of uncontrolled, recurring rebirth and death in cyclic existence.
Hope this answers the question in a relatively concise fashion.