A sensor type (Literally meaning: perception type) is a ninja that is capable of detecting the presence of other people by directly sensing their chakra.
A skilled sensor can detect their targets from a great distance, as well as differentiate persons by their chakra signature alone. They are even able to tell when other sensors are using their abilities. Karin is able to tell exactly who is among a crowd of people and her sensory skills are such that she is even able to tell if her targets are lying based on any fluctuation of their chakra. Mū and Tobirama Senju have displayed that sensors can discern between chakra signatures of shinobi from different populations and even link them back to relatives; for example Mū commented that Gaara and his father Rasa share similar chakra signatures, he also noted that different chakra signatures from different villages were gathered in one place; while Tobirama was able to discern that Karin was descended from the Uzumaki. Nagato, another skilled sensor, was able to detect the exact location of the person controlling them despite that person erecting a barrier around themselves. Members of the Yamanaka clan have even displayed a technique whereby sensory perception can be transferred to other non-sensors.
Conversely, there are methods that can hinder a sensor's ability to detect individuals. Mū has mastered the Dustless Bewildering Cover technique to such an extent that he is able avoid detection by traditional sensory techniques, but can be sensed through other means such as contact-type sensory techniques. Karin is able to completely suppress her chakra, though she cannot use her abilities while doing this. The Aburame clan members can use their insects to create false chakra signals and confuse sensors. The Demonic Illusion: Steaming Multistoried Building used by the Second Mizukage and his Giant Clam also prevents them from being sensed through traditional methods while in effect. Zetsu is also able — through unknown means — to render himself undetectable.
Throughout the series, various characters have commented on the "feel" of a person's chakra and similar comments regarding the ability to sense high levels of chakra a little, leading to the possibility that chakra sensing is merely a more refined version of this by either teaching or being simply born more sensitive. Several other methods of "sensing" have also been displayed such as Gaara's Concealed Sand Picture Cat technique, which is a contact type (接触タイプ, sesshoku taipu) sensory ability as well as Kidōmaru's Spider Web Area and Nagato's Rain Tiger at Will Technique.
According to Karin in the anime, sensor types have different specialities depending on their villages: Konoha shinobi often use ninken with their smell and tracking sense. Karin's sensing abilities are derived from her Mind's Eye of the Kagura; it is unknown if other sensors also have a specific techniques to do so, or if it is just a general skill.
There are also other methods of sensing, Tobirama Senju and Minato Namikaze were able to detect enemy presence and numbers by simply touching the ground with their index finger. Tobirama went as far as being able to tell that Orochimaru's new body was made almost entirely of Hashirama's DNA. Gaara also possess a sensory ability which is tactile in nature: Concealed Sand Picture Cat, which allows him to locate targets that his sand comes into contact with, even if they are invisible.
Sensor types can also detect changes in a person's personality by sensing their chakra as chakra reflects the user's personality. To be able to sense an opponent, a shinobi must have their chakra to "sensory mode". As such, if they are distracted and don't have their chakra on sensory mode, they can be kept from detecting enemies.