You'll often see a series of eyepieces advertised as being "parfocal". This simply means that when you switch to a different eyepiece in the set, you can take out one eyepiece and insert another without having to refocus. The hooker is that they all have to be part of the same set. Manufacturers make eyepieces parfocal by simply varying the barrel length so that the focal plane of every one in the set is always positioned at the same place in the focuser tube of the telescope, regardless of the focal length of the individual eyepiece. Eyepieces by different manufacturers may end up being just about parfocal with each other, but that's pure happenstance!