If someone enters into or onto your property without your permission then that is trespass. So if I stand at your gate and you see me and welcome me in then that is not trespass. However people generally do not object to callers coming up the garden path (if they have one) for the purposes of delivering a letter or making enquiries (meter readers, census takers etc) and so these types of callers have standing consent to act in this way according to the law.
However if you had a sign up saying (for instance) no pizza leaflet deliverers allowed beyond this point, and such a person ignored the notice to enter your property then that would again be trespass.
Furthermore you can withdraw your consent to a person who has entered onto your property with your consent (express or implied).
An example: a lady with a clip-board recently called at our door. It turned out she was trying to get people (like me) to sign up for loft insulation.
I soon said I wasn't interested but she was very, VERY pushy and after threatening to call the police I had to close the door in her face.
What else, legally, could I have done? First, I might have sued her for trespass - a bit daft as she would have gone long before I could commence proceedings.
The - simpler - alternative would be to remove her physically from the premises, or have someone else do it for me. This is because you are legally entitled to use reasonable force to remove trespassers from your property.