Scouter

A Scouter is an all purpose computer that Frieza's army use, Scouters are mainly used to measure power levels early in Dragon Ball Z.
According to the anime, scouters were originally invented by the Tuffles until the Saiyans exterminated them and stole their technology. From the Saiyans, scouters were introduced to the Planet Trade Organization. During the series, almost all of the Planet Trade Organization soldiers can be seen wearing one. 
In the series, a scouter's primary purpose is to calculate the individual power level or ki concentration of biological characters by converting the readings into numbers. It is never explained what units of measure are used to read ki levels; Akira Toriyama simply stated in an interview that it could not be done by.
Scouters also serve the purpose of gathering information, and for interstellar communication, as seen when Raditz sent Piccolo's description of the Dragon Balls to Nappa and Vegeta, as well as Frieza learning of the existence of the Namekian Dragon Balls from eavesdropping on Vegeta's conversation with Nappa. Vegeta also used his scouter to keep track of the time after he said he and Nappa would give the Z Fighters three hours for Goku to arrive. Scouters can also tell the location of someone, as demonstrated when members of the Planet Trade Organization use scouters to hunt down enemies. 
Though it is possible to sense others' ki without advanced machinery, as the Z Fighters do, scouters provide the ability to read power levels to less-trained fighters. Scouters do have some disadvantages, however. Their main drawback is that a trained person is capable of consciously lowering their ki (see Power Up), and can therefore fool scouter users into thinking they have a lower power level than their real level. Vegeta speculates that is how Raditz was killed, and this later proves a key component in the downfalls of several of Frieza's notable soldiers. 
Another disadvantage is that scouters may explode on certain conditions. The reason for this is not mentioned explicitly in the anime or the manga, but the recorded cases have happened when the scouter is detecting a high power level or when a power level is rapidly increasing, possibly overloading it. Dodoria suggested that it was a problem with older scouter models. There are two generations of scouters:[3] the first generation ones (such as Raditz's and Zarbon's) explode when reading a power level over 22,000, something that is corrected in the improved second generation scouters (such as Dodoria's and the ones used by the Ginyu Force) which are always in development to go along with battle power inflation,[4] at least up to 180,000. An example of scouters exploding while someones power level is rising rapidly would be that which occurred to Bulma's (formerly Raditz's) scouter when Goku was fighting Vegeta for the first time. The explosion took place around the time that Goku had used the Kaio-ken times 3, causing his power level to rise at an incredible rate and thus overloading the scouter. Even the second generation scouters can be overloaded and explode, as Frieza's did reading Vegeta's power.