Eurorack is a type of modular synthesiser. In a non-modular synthesiser the various functional blocks that generate and modify sound (such as oscillators and filters) are preconfigured — they are connected in a predetermined way. In a modular synth these blocks are connected manually using patch cables. This makes them harder to use but more flexible. It also makes them expandable; if you need another oscillator you just buy another oscillator module. There are several different modular synth formats. Eurorack is popular because it is open, the modules are physically small and there are many hundreds of modules to choose from, from a large number of manufacturers.