In Nuclear physics, this type of decay transforms a proton into a neutron (or vise versa). It will do this in unstable nuclei in an attempt to stabilize the nucleus. This is also called and electron emission.
The neutron decays into a proton, and electron, and an electron neutrino. As you can see, the mass number remains the same, since you traded a neutron for a proton. As you change the proton number (the atomic number) you change the element itself.
The newly produced electron and its neutrino fly off in opposite directions.