It is the same as "Hi" and frequently used as an alternative to "Hi" when it is someone you are glad to see. (It is too friendly for formal settings, or someone tht you do not feel good about) Originally it may have been a question (How do you do?) but it is not a question now.

Howdy is a very casual form of hello. typically used by friends. however, you would find this to be used in the rural/farming focused areas, it's more of a "cowboy" type greeting. this really is not used so much anymore. An example would be: Howdy Ahmed, (said very casually) Good to see you again. How have you been?