blender Getting started with blender Hello World! (Add-On)

Example

# not all of this is required, but just here for reference
bl_info = {
    "name": "Hello World",                          # name of the add-on
    "author": "Blender developer",                  # name of the author
    "version": (1, 0),                              # version number for the add-on
    "blender": (2, 78, 0),                          # version of Blender the add-on is compatible with
    "location": "Space > Hello World",              # where the user can find the add-on
    "description": "Greets something",              # add-on description
    "warning": "Beta version",                      # whatever the user needs to be warned about
    "wiki_url": "",                                 # documentation link
    "category": "Development",                      # add-on category
    }

# the blender python module
import bpy
# this is just for convenience - could just use as bpy.props.StringProperty, but there are normally lots of properties
from bpy.props import StringProperty


class HelloWorld(bpy.types.Operator):
    """Says hello to the world."""       # python docstring 
    bl_idname = "greetings.hello_world"  # this will be callable with bpy.ops.greetings.hello_world()
    bl_label = "Hello World"             # the user-friendly name for this operator (e.g., in a button)
    bl_options = {'REGISTER', 'UNDO'}    # 'UNDO' is only here for reference (you can't actually take back what you say)

    name = StringProperty(
            name="name",
            default="world",
            description="Who to say hello to",
            )

    def execute(self, context):
        # Note: The execute method is called when the user clicks on the operator or calls it from python.

        message = "Hello, " + self.name + "!"
        
        # print to console
        print(message)
        
        # show a popup that automatically goes away (in info area's header)
        self.report({'INFO'}, message)
        
        # show a popup under the cursor that doesn't go away automatically
        self.report({'ERROR'}, message)

        # generally return {'FINISHED'} or {'CANCELLED'} at the end of the execute method
        return {'FINISHED'}


# this is automatically called when the add-on is enabled
def register():
    # simply register the class
    bpy.utils.register_class(HelloWorld)
    

# this is automatically called when the add-on is disabled
def unregister():
    # simply unregister the class
    bpy.utils.unregister_class(HelloWorld)


# common "pythonic" approach to main()...call register() automatically
if __name__ == "__main__":
    register()

Save this in an python file (.py), then install as a regular add-on in Blender. Type "space" in just about any area in Blender and type "Hello World" to find the operator you built.