E1 (or E-1) is a European digital transmission format devised by the ITU-TS and given the name by the Conference of European Postal and Telecommunication Administration (CEPT). It's the equivalent of the North American T-carrier system format. E2 through E5 are carriers in increasing multiples of the E1 format.
The E1 signal format carries data at a rate of 2.048 million bits per second and can carry 32 channels of 64 Kbps* each. E1 carries at a somewhat higher data rate than T-1 (which carries 1.544 million bits per second) because, unlike T-1, it does not do bit-robbing and all eight bits per channel are used to code the signal. E1 and T-1 can be interconnected for international use.
E2 (E-2) is a line that carries four multiplexed E1 signals with a data rate of 8.448 million bits per second.
E3 (E-3) carries 16 E1 signals with a data rate of 34.368 million bits per second.
E4 (E-4) carries four E3 channels with a data rate of 139.264 million bits per second.
E5 (E-5) carries four E4 channels with a data rate of 565.148 million bits per second.
* In international English outside the U.S., the equivalent usage is "kbps" or "kbits s-1."