There are various terrain types in both the strategic level and also the combat level
Strategic map terrain
Strategic terrain can also impact sensors which is covered on the Sensors page
Asteroid Belt
Ring System
Accretion Disk
Nebula
Magnetic Field
Speed reduced by 35%
Deep Hyperspace
This is denoted by purple clouds that occur on the strategic hyperspace map
Hyperspace Storm
This is denoted by purple Deep Hyperspace clouds that are brightly lit up
Combat map terrain
Asteroids
Asteroids are individual, destructible objects on the battlefields that can block most weapons fire and most movement (excepting fighters and phased ships)
Nebula
Nebula incur a malus of -30%/-20%/-10%/0% speed. This is calculated after any zero flux speed boost is applied. Do note that level 1 of the Navigation skill to reduce terrain penalties does not apply to combat map nebula