what is the process of synapsis?

Answer 1

Synapsis happens in prophase I of meiosis I. The homologous chromosomes pair up and match up all along their length. Chromosome number one finds the other number one and lines up with it, and so on. See a picture here:

http://io.uwinnipeg.ca/~simmons/15cldv3.htm about halfway down on the page.

Crossing over happens while the chromosomes are synapsed.
The synapsed chromosomes are called a tetrad because tetra means four and there are four sister chromatids in the set.