The first three missions were based on the model 3MV planetary probe, intended to explore Venus and Mars. After two failures, Zond 3 was sent on a test mission, photographing the far side of the Moon (only the second spacecraft to do so) and continuing out to the orbit of Mars in order to test telemetry and spacecraft systems.
The missions 4 through 8 were test flights for manned circumlunar flight. The Soyuz 7K-L1 (also mentioned just as L1) spacecraft was used for the moon-aimed missions, stripped down to make it possible to launch around the moon from the Earth. They were launched on the Proton rocket which was just powerful enough to send the Zond on a free-return trajectory around the moon without going into lunar orbit (the same path that Apollo 13 flew in its emergency abort). It could have carried 1 or 2 cosmonauts.
There were serious reliability problems with both the new Proton rocket and the new Soyuz, but the test flights pressed ahead with some glitches. The unmanned circumlunar Zond 5 flight in September 1968 was part of the reason NASA flew Apollo 8 to the moon in December 1968 instead of the Earth orbital test which had been planned, because the CIA believed the Soviets were planning a human flight next. Had Apollo 8 not flown when it did, it is possible the Russians would have been the first to fly around the moon in late 1968 or early 1969. However, four of these five Zond flights suffered malfunctions that would have injured or killed any crew.
Instrumentation flown on these missions gathered data on micrometeor flux, solar and cosmic rays, magnetic fields, radio emissions, and solar wind. Biological payloads were also flown and many photographs were taken.