What and when was the "recombination era"?

Answer 1

this refers to the time 380 000 years after the big bang when the universe had cooled enough for electrons to combine with nuclei. photons could pass freely without being scattered so the universe became transparent. before this, the universe was much like a heavy fog. today, we see the light from this "surface of last scatttering" stretched to microwave wavelengths and call it the cosmic microwave background.

look here:
http://en.wikipedia.org/wiki/Big_Bang#Cosmic_microwave_background_radiation

Answer 2

There's a misnomer commonly used in cosmology: "Recombination Era". That term refers to the time, about half a million years or so after the Big Bang, when the expanding universe cooled down enough for atoms to form. As the temperature dropped below a few thousand degrees electrons were finally able to attach themselves to nuclei; before that they roamed freely in a plasma. Free-electron plasmas are quite good at scattering light. It's impossible to see very far through them.

Suddenly, at the Recombination Era the cosmos became transparent. Photons decoupled from matter. The fog lifted.

Answer 3

Recombination Era

Answer 4

These links explain it much, much better than I ever could: