The so called Sporadic E (Es) is the result of a ionization in the E layer, which is located in the ionosphere. This propagation is very common during the summer period but can actually appear any time of the year. It is also most common during the time when the sun is up but can, during extremely strong ionization, be there throughout the night. Normal distances covered are 800-2400 kms. During very active periods the signal can when landing after the bounce on the E layer start on a 2nd, 3rd, 4th, 5th and 6th bounce too! As a result large parts of the earth can be worked with a lot of luck. One of the down sides with Es is that the reflecting area in the ionosphere can be very small leaving you with only noise while your friend only a few kilometers away reports the DX being strong as a local. As the area moves you will mostly also get your chance to hear that station. This behavior helps the stations that are not so well equipped to work the distant rare stations as they might be the only station audible at that moment. Conclusion: To be active when things happen is key, not how powerful your station is!
Some other things worth to note is that when the Es is strong you can actually get backscatter which makes it possible to work stations much closer than normal. You can also hear backscatter from more than one direction (I have hears stations peaking from 3 different directions at the same time!) As you will read under TEP sometimes the Es can be your propagation mode for the first 1000-2000 kms and the second bounce will be a link to TEP. For us in northern Europe this is quite often a must if you want to work Africa south of the equator.
Francois has given me a lot of rare wet squares,
via Es, during his travels across the oceans