Kraftwerk was one of the first groups to adopt the synthesizer as central to their sound. Prior to the 1970s, most pop and rock groups were guitar based and only used synths to augment their sound. However, Kraftwerk built much of their music from this period, including "The Robots," around the synthesizer as a central instrument.
Vocoder is short for voice encoder. Originally developed in the 1930s to encrypt voice transmissions for security purposes, the vocoder was adapted by avant-garde-minded musicians in the 1960s and 1970s. Kraftwerk used vocoders for the distinct, robotic lead vocal sound heard on "The Robots" and much of their music from this era.
Electronic drums are essentially pads that can be programmed with different rhythmic sounds and tones and struck in the same manner as an acoustic drum set. They lend "The Robots" its distinctive electronic beat. In the video for the song, you can see the band striking the drums with regular wooden drumsticks.
Sequencers are electronic devices or computer programs that play back sounds and tones, often in rigid, repetitive patterns like those heard in "The Robots." Early sequencers, such as those used by Kraftwerk on "The Man-Machine," generally had 16-button grids with each button being 1/16th of a measure.