The word NEO stands for Near Earth Object, meaning a minor body of the solar system (or in other words a comet or an asteroid) which comes into the Earth neighborhood.
A first classification of NEOs divides NEC (Near earth comets) from NEAs, Near Earth Asteroids.
NEAs constitute the vast majority of NEOs and are further divided into three main families, depending on the features of their orbits. In particular they are classified in three groups (Amor, Apollo and Atens) according to their perihelion and aphelion distances and their semi major axes.

In this image you can see the Earth's orbit (in blue ) and the classical shape of the obits of the three main classes of NEAs.

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