Source: Next to / Beside

" "next to" is the most general-purpose of the group. It indicates that things or people are quite close laterally -- from right to left. Any person or thing can be next to just about any other person or thing. "next to" is not generally used for vertical spatial patterns.

The church is next to the hospital.
The fork is next to the knife.
John is next to Mary.
The shelves are next to the wall.

*The door is next to the floor. (* indicates something anomalous. Here, the spatial relation is vertical.)

"beside" is almost the same as "next to", but is somewhat higher in register, in my opinion. I rarely use the word "beside", preferring "next to". Also, "beside" is awkward with things that have no obvious sides, like walls.

The church is beside the hospital.
The fork is beside the knife.
John is beside Mary.
?The shelves are beside the wall.
*The lamp is beside the ceiling. "

Hope it helps you!
Have a nice day! Abigail.