Star Trek: Every Borg Ship Explained

With a diameter of 600 metres and a standard complement of 11,000 drones (according to Memory Alpha), Borg Spheres are significantly smaller vessels than Cubes, but with a very similar design.

Spheres, like Cubes, are used for assimilation primarily, but also have the ability to time travel. During The Battle Of Sector 001 (in Star Trek: First Contact), the invading Cube had a Sphere inside which launched from the Cube right before it exploded, then traveled to Earth's past to assimilate humanity in the 21st century. It's not known if any other Borg vessels have this power as we never see the Borg use this tactic again, but it's likely that time travel is commonplace for the Collective.

The Sphere from First Contact was a form of escape pod, saving the Borg Queen and giving the Borg another chance after the Cube was unexpectedly defeated. It's very likely that other Cubes also have a Sphere inside as a last resort.