We have imaged the ejecta of GK Persei (Nova Persei 1901 A.D.) with the Hubble Space Telescope (HST), whose 0.1arcsec resolution reveals hundreds of cometary-like structures with long axes aligned toward GK Per. One or both ends of the structures often show a brightness enhancement relative to the structures' middle sections, but there is no simple regularity to their morphologies (in contrast with, for example, the Helix nebula). Some of structures' morphologies suggest the presence of slow-moving or stationary material with which the ejecta is colliding, while others suggest shaping from a wind emanating from GK Per itself. The most detailed expansion map of any classical nova's ejecta was created by comparing HST images taken in successive years. Wide Field and Planetary Camera2 narrowband images and Space Telescope Imaging Spectrograph spectra demonstrate that the physical conditions in this nova's ejecta vary strongly on spatial scales much smaller than those of the ejecta. Directly measuring accurate densities and compositions, and hence masses of this and other nova shells, will demand data at least as resolved spatially as those presented here. The filling factor of the ejecta is 1% or less, and the nova ejecta mass must be less than 10 -4 M . A modest fraction of the emission nebulosities vary in brightness by up to a factor of two on timescales of one year. Finally, we present the deepest images yet obtained of a jet-like feature outside the main body of GK Per nebulosity, and the first spectrum of that feature. Dominated by strong, narrow emission lines of [N II], [O II], [O III], and [S II], this feature is probably a shock due to ejected material running into stationary interstellar matter, slowly moving ejecta from a previous nova episode, or circumbinary matter present before 1901. An upper limit to the mass of the jet is of order a few times 10 -6 M . If the jet mass is close to this limit then the jet might be an important, or even dominant mass sink from the binary system. The jet's faintness suggests that similar features could easily have been missed in other cataclysmic binaries.