Many times i get confused whether i should use 's with a noun or not. For example:

Engine parts or Engine's parts
Human brain or Human's brain
Wordpress plugin or Wordpress' plugin
Computer program or Computer's program etc.

I want to know why and when we use one or the other. What you call such a group of words? Is it phrase? It should be noted that English is not my native language so pardon my ignorance.

Many times i get confused whether i should use 's with a noun or not. For example:

• Engine parts or Engine's parts
• Human brain or Human's brain
• Wordpress plugin or Wordpress' plugin
• Computer program or Computer's program etc.

I want to know why and when we use one or the other. What do you call such a group of words? Is it a phrase? 
It should be noted that English is not my native language so pardon my ignorance.