Land scouts only exist in tech 1. They are the fastest land-based units, and also the cheapest. They all have radar and some have basic weapons; the Cybran land scout has cloaking instead. They all have extremely low health and because of this they are not recommended for attacking by themselves. They can be used for early game scouting and they can destroy unprotected T1 mass extractors or engineers, the latter being more dangerous due to the engineers reclaim ability.
Another great use for scouts is sending them with artillery (or any other unit whose weapon out-ranges its vision) in battle. This gives the artillery an advantage because the scouts radar allows the artillery to shoot farther.
For a complete list of land scout units, see the land scout units category.