E-6 is a designation for the process used to develop color slide film. Color slide films typically have brand names of the form something-Chrome.
Because conventional films develop as a negative image, slides require reversal processing and are sometimes termed "reversal films." The E-6 reversal process results in positive color image (light and dark appear normally), which can be viewed or projected directly.
E-6 is a standardized chemical sequence which does not vary with film speed or brand. Every film designed for E-6 processing goes though its chemical steps at equal temperatures and timings.
Slide films and E-6 processing have dropped in popularity to the point where E-6 is mainly offered by specialty labs. Some may offer push processing to a higher effective film speed as one of their E-6 services (at an extra charge).
The other common color development chemistry used today is the one for color negative film, process C-41. When a film designed for E-6 processing is deliberately put through C-41 instead, this is termed cross processing.