  The number of known periodic variables has grown rapidly in recent years.
Thanks to its large field of view and faint limiting magnitude, the Zwicky
Transient Facility (ZTF) offers a unique opportunity to detect variable stars
in the northern sky. Here, we exploit ZTF Data Release 2 (DR2) to search for
and classify variables down to r ~ 20.6 mag. We classify 781,602 periodic
variables into 11 main types using an improved classification method.
Comparison with previously published catalogs shows that 621,702 objects
(79.5%) are newly discovered or newly classified, including ~700 Cepheids,
~5000 RR Lyrae stars, ~15,000 Delta Scuti variables, ~350,000 eclipsing
binaries, ~100,000 long-period variables, and about 150,000 rotational
variables. The typical misclassification rate and period accuracy are on the
order of 2% and 99%, respectively. 74% of our variables are located at Galactic
latitudes, $|b|<10^\circ$. This large sample of Cepheids, RR Lyrae, Delta Scuti
stars, and contact (EW-type) eclipsing binaries is helpful to investigate the
Galaxy's disk structure and evolution with an improved completeness, areal
coverage, and age resolution. Specifically, the northern warp and the disk's
edge at distances of 15--20 kpc are significantly better covered than
previously. Among rotational variables, RS Canum Venaticorum and BY
Draconis-type variables can be separated easily. Our knowledge of stellar
chromospheric activity would benefit greatly from a statistical analysis of
these types of variables.
