Hi Omar, stock and broth are the same. It is the flavored liquid from boiling animal products, seafood or vegetables in water. It is the base to many soups. Examples: chicken both and chicken stock (same thing)

soup is the hot liquid based food. 

Gravy is a type of sauce often made from meat broth. 

Just one thing to add to the previous answers:

Although stock and broth are essentially the same, I think there is a difference.

Broth is always liquid, and it can be a very simple dish in its own right. Especially in the past, broth was often given to invalids who had little appetite but needed to build up their strength. Sometimes poor people might drink broth as a source of protein, perhaps with the addition of bread or pasta to give it bulk.

Stock is essentially a cooking ingredient used as a base for various dishes. For most people, it comes in the form of a dry store-cupboard basic, often as a concentrated powder or cube. You add hot liquid to this to make a quicker (but inferior) version of the stock you might make from scratch with meat and/or vegetables. Stock is never a dish in its own right.

As the others have explained, soup is a complete dish or simple meal (based on stock or broth but thickened, with the addition of vegetables and sometimes other ingredients such as rice or pasta). Gravy a meat-based sauce used to add flavour and moisture, often poured over or served alongside a dish.