When you are first learning, you will keep the bow on the string while playing a sequence of notes, but obviously you change the direction of the bow for each note. This is called 'detache' bowing.
More advanced techniques involve lifting the bow between notes and even using a controlled bounce to separate the notes.
After my first reply it occurred to me that I should have added a recommendation on bowing practicing. I think it can be very helpful to bow whole notes on the open strings. That is, start at the frog and bow down all the way to the tip. Keeping the bow on the string, change directions and bow up from the tip to the frog. Do this several times and then repeat on the other strings. Try to keep the bow at 90 degrees to the strings the whole time.