What the hell is 403B and how is it different than 401(k)?

I had my paycheck arrived to my bank account today. I noticed a 2.5% reduction because of 403B. So I did a research online today and apparently this is something that my employer is taking from my paycheck and investing it into some random fund company. <br><br>So I logged into the website online and saw that it is ~2.5% from my paycheck. I was told from my HR that this is a new policy and it is not mandatory to continue paying this 2.5% but I need to fill out a paper for them to stop taking this. I don't know what to do. <br><br>So what is 403B and how is it different from 401(k)? I had a 401(k) plan from my previous job but never put any money in it. So 401(k) is from my previous job and 403B is from my new one. What is the difference? Do you think I should keep contributing?