Answer 1

The work of tester is to test (or check), if the program works well. If not crashes, if outputs takes correct results, etc.
This seems like a simply work, but is for precise people. At begin must be prepared a plan of hunderds tests over the whole application. And then check, all of them, one by one. Log every check, and when the bug is found, there must be a briefly decribed. When the developers make change of code, then some tests must be repeated.
In some cases there can be used a tool for automatic testing, So the work of program-tester can be to design and code it with script language..