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A computer based catalog of UBVRI photoelectric photometry of T Tauri stars has been compiled for the purpose of studying their variability. It presently includes over 10,000 entries on 80 stars and will be updated on a regular basis; it is available on Internet. The catalog is used to analyze the light variations of T Tauri stars in an attempt to illuminate the nature and causes of the phenomena. We propose a framework for interpreting their behavior which attributes the V light variations to only two causes: the rotational modulation of a star with cool spots on its surface and accretion from a circumstellar disk. If flares play a role, it is only in the U (and possibly B) filter data. It is useful to divide the stars into three groups according to their spectral properties; weak T Tauri stars (WTTS), classical T Tauri stars (CTTS) and early-type T Tauri stars (ETTS; here defined as spectral class of K0 or earlier). Three distinct types of variability are displayed by stars in the catalog. Type I variations are periodic and caused by the rotational modulation of stars with cool spots. Type II variations are caused by hot spots and, it is argued, are the result of changes in the excess or ``veiling" continuum driven by accretion. It is shown that a third type of variation exists which is displayed by the ETTS. These stars can have very large amplitude variations ($>$2.8 mag in V) without showing any evidence of a ``veiling" continuum. It is argued that Type III variations also result from the accretion process which requires that the accretion luminosity of large amplitude variable ETTS is often masquerading as the stellar photosphere. Our interpretation suggests that the accretion energy of CTTS is released in a large number of small, hot spots which appear and disappear on a timescale of hours to days whereas on the ETTS the accretion zones are larger and variations are on a timescale of days to weeks.
