Sensors are devices capable of detecting a variety of objects and activity, which vary between races.
Ancient long range sensors were used on ships to detect other vessels and planets. The long range sensors on City-ships can scan massive distances, at least a quarter of a galaxy, and are one of the few ones known to be able to detect ships in hyperspace. Other versions - which were presumably less powerful - were installed on Aurora-class battleships and other vessels of similar status that have a shorter range.
Short-range sensors scan in much more detail, but at a far shorter range. City-ships possess sensors that can scan all life forms on a planet's surface (and probably in orbit over the city,) and identify them. They can also create visual images based on the sensor data, similar to the way Asgard sensors allow projection and interaction of holographic avatars over great distances, although this has only been seen done through a Control chair as it may require the users mind to interpret some details in a similar way to the virtual reality pods on P7J-989. The sensors also appear to be able to detect sound, as the Lord Protector of The Tower claimed to know what Lt. Colonel John Sheppard's team was saying. (SGA: "The Tower")
Ancient life signs detectors can identify solid objects, life forms, and energy sources. They can also scan for the Ancient Technology Activation gene and possess the ability to scan underground in the same radius as the size of Atlantis. (SGA: "The Tower")
Asgard sensors are highly advanced. They can scan through Goa'uld shields and pinpoint life signs from far away. They are also what allows their beaming technology on board their ships to lock on to an object so accurately, it can even successfully beam a goa'uld symbiot out of a host. A less advanced version of this technology was installed on the Earth ships of the Daedalus-class, although it would have been upgraded to the most advanced version as part of the Asgard legacy. (SG1: "Unending")
Save for Atlantis's sensor array, Asgard sensors seem to be the only ones capable of scanning hyperspace without the scanning ship entering it. Whether they are able to scan normal space from hyperspace is unclear.
The Goa'uld possess sensors that are less advanced compared to the Ancient and Asgard counterparts. When Anubis ascended, he was able to gain access to Ancient technology and upgrade the sensors on his vessels to the point where they could detect a cloaked Tel'tak. (SG1: "Redemption, Part 2")
While the Tau'ri do have sensor technology of their own (radar, etc.), they mostly rely on a less advanced form of Asgard sensor technology. Such sensors are located on Daedalus-class ships at the top-rear port section of the ship. They are used for detecting life signs, locator beacons, as well as other ships in space. However, after the Tau'ri received all of the knowledge of the Asgard, it is possible that they have updated their ships with a more advanced version of Asgard sensors, and maybe even applied information found in the Atlantis database as well. (SG1: "Unending")