What is exactly and Intro? ...And an Outro?

One is tempted to describe the introduction as the first section of a song. The problem is that "Yellow Submarine" begins with a section that must be clearly classified as a verse. Therefore, a better definition would be:

The introduction —or intro— is a section that appears at the beginning of a song, and it is not repeated.

Most songs possess and introduction, but —as "Yellow Submarine" is showing us— the introduction is not compulsory.

The introduction can be as sort as a single chord, or last several minutes.

Introductions are erasable. They can be removed without affecting the essence of the song, which is located normally in the chorus.

What is a coda?

We would be tempted to describe the coda —or outro— as the last section in a song. But, for the sake of consistency, this is not a suitable definition. Rather, we should define the coda as the last section of a song, provided that it has not been presented previously.

The problem is that, using this more restricted definition of coda, we are left with very few songs possessing a genuine coda, because, most times, the coda is built from recycled materials previously introduced. Typically, the coda is made out of recycled chorus, as it happens in "I`ve Just Seen a Face" and "Yellow Submarine."

Nevertheless, I prefer to use this more restricted definition of coda, and describe it as the last section of a song, whenever it is a unique section that has not been presented previously in the song.
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This more narrow definition of coda helps to better illustrate the simplicity of many songs. For example, under this definition "Yellow Submarine" must be described as:

V-V-C-V-C-V(I)-V-Cn

rather than as

V-V-C-V-C-V(I)-V-O

The notation V-V-C-V-C-V(I)-V-Cn captures the fact that "Yellow Submarine" contains only two different sections, verse and chorus.