Hi Walter!
Disney only sells full day passes to the parks, so you'll still need to purchase a full day ticket even if arriving later. Depending on your stay would be the ticket length. So if you're staying 2 days, you'd get a 2 day base ticket, which entitles you to 2 full day access to one of the 4 theme parks (Magic Kingdom, Animal Kingdom, Epcot, or Hollywood Studios). If you add the Park Hopper option, then you can go "theme park bouncing" from park to park throughout the day.
My suggestion: If you're arriving late, and not staying long (3 days or less at Disney), try to plan which Park you may do the least at, and use that Park as your trip when you arrive. Or, see if one of the parks is doing Extra Magic Hours for that evening, and go to that one to give you more time to have fun :-)
May you have a magical time at Disney!