The Daedalus traveling through hyperspace
Hyperspace is an alternate dimension used by several races for faster than light travel accessible by means of a hyperdrive.
Overview
Entry into hyperspace is achieved when a vessel opens a subspace window, which allows a vessel to enter subspace and achieve faster-than-light velocities. While at its core all hyperdrive technology followed the same basics, the hyperdrive technology of each race was ultimately unique in the sense that they each produced a unique type of subspace frequency. If a subspace frequency would be disrupted, vessels are destroyed upon entry into hyperspace; other hyperdrives would remain uneffected due to the differences in frequency however. (SGA: "First Contact")
Hyperspace apparently gives off high levels of radiation, shown by hull degeneration on Wraith Hive ships. With the exception of the Asgard core, which when activated can be tracked in hyperspace by Ori ships, the only method of detecting objects within subspace is via the long range sensors of Atlantis (and likely other city-ships as well) and Asgard sensors. (SG1: "Small Victories", "Unending")