The EI is nothing more or less than the speed that you choose. Most films today have a bar code on the cassette called the DX code. Most modern automatic cameras read this bar code and set the film speed automatically. Another fundamental concept is sensitization. Most films today are panchromatic, which means that they are sensitized to respond to all colors of the visible spectrum.
Sensitization is achieved by dyes in the emulsion layers.
Other films such as Ilford's extended red SFX 200 or the infrared films from Kodak, Konica, and Maco have red-sensitive dyes which respond to the near infrared area of the spectrum, an area where the human eye is not sensitive. This gives an other-worldly effect to your pictures providing you use the appropriate filtration. For instance, Kodak T-Max films exhibit reduced blue sensitivity and increased orange sensitivity as compared with Ilford Delta films. In color, there are films for specific lighting conditions, too. Daylight-balanced film is perfect for outdoor or flash lighting, but will give very odd yellow color rendition under tungsten light unless a filter is used.