There are two principal types of orchestration which are used according to the nature of the composition itself. If the work (or specific part of it) consists of a melody with its accompaniment, it will be orchestrated as two distinct and separate parts. If on the other hand, it consists of a choral, harmonic-like mass of sound, it will be orchestrated in an even, uniform manner.

Melody - Accompaniment

This form of music can be orchestrated in a considerable variety of ways -
     the melody can be given to one or several soloists, in unisson or in octaves,
          or to a complete orchestral section, also in unisson or in octaves;
     the accompaniment can be divided into sustained and rhythmical elements,
          which can be distributed in various ways.
The opening of the Tchaikovsky Piano Concerto No. 1, after the short introduction,
          is an excellent example of this.
     Description

Layers

When a work (or section thereof) consists of a uniform mass of sound, it should preferably be orchestrated in layers. Each layer will consist of only one color or family of orchestration and will englobe all the material of the work, a musical entity in itself. This is usually preferable to dividing the musical mass into different orchestral colors.

Examples
1. With brass instruments, rather than orchestrate a 4-voice choral passage
          with 1 trumpet, 2 horns, and 1 trombone,
     it is preferable (if the range permits) to make 2 layers -
          one layer with 2 trumpets and 2 trombones, and
          another layer with 4 horns.
2. With woodwind instruments, rather than orchestrate a choral passage
          with one layer using a mixture of all of them,
     it is preferable to make several homogeneous layers.