Among the nearby stars, the properties of Barnard's Star (GJ 699) make it a prime target for the search of Earth-like planets using ultra-stable spectrographs. This search requires a very precise characterization of the stellar activity and the magnetic cycle of the star since these phenomena induce radial velocity signals that can be misinterpreted as planetary signals. We have carried out a characterization of these phenomena using a set of spectroscopic data that covers about 14.5 years and comes from seven different spectrographs: HARPS, HARPS-N, CARMENES, HIRES, UVES, APF, and PFS; and a set of photometric data that covers about 15.1 years and comes from four different photometric sources: ASAS, FCAPT-RCT, AAVSO, and SNO. Our analysis reveals a rotation period of 145 ± 15 days, consistent with the expected rotation period according to the low activity level of the star and previous claims. We also find evidence of a long-term cycle of 10 ± 2 years that is consistent with previous estimates of magnetic cycles from photometric time series in other M stars of similar activity levels. Finally, the numerous measurements from high-precision radial-velocity instruments reveal the presence of a low-amplitude periodic signal with a period of 233 days that is best explained as arising from a planetary companion. The candidate planet around Barnard's star is a cold super-Earth, with a minimum mass of 3.2 times that of Earth, orbiting near its snow line (the minimum distance from the star at which volatile compounds could condense).