1) Do you guys write a lot of automation skripts ?
Yes
2)How is the day to day task ?
Depends. Mix of writing code, runing the test, manual testing, understanding the requirements, talk to the BA ,PM, Developer so on and so forth.
3)Is automation skripts too complicated to understand compare to actual application code for a newbie ?
Depends. Selenium is primarily used for Black box testing( Testing the end product and not the code). Therefore you are basically testing for interaction of the webpages. example you would write a code to click a button and check for the expected results on what happens. In order to do that you would find the locator of the button using ID, Xpath etc and send the command using selenium interface to click it.
Selenium commands itself are not hard to understand.
How much knowledge do we need in Java programming . Is Core Java sufficient ?
You dont neccesrily need to know Java programming. You could used C#,Javaskript, PHP, Ruby etc. You would defintely need to know how to program in order to be a automation tester. You dont have to know all the concepts in programming but you need to know classes, methods, arrarys, iteration, static concepts and so on. I guess you can google on what would you need to learn to be a automated tester from a programming perpective.
Lastly, automation is just not Selenium and some programing langange. There are different concepts and other frameworks used to make a complete testing framework.