Java !!! I am Confused!!

About a week when i got interested in web development and watched those microsoft tutorial videos on HTML5 and learnt about HTML5 and CSS3. The tutorials included JQueries (obviously with web development comes java).

I already had a bit of knowledge about java (netbeans) from my previous semester which actually did not go hand in hand with those videos. I started getting confused.

Then just today i thought of giving greasemonkey scripts a try as they also had java. But again i ended up wasting my whole day and frustated.

Please tell me are there different versions of java in different scenarios and if yes from where should i started as i think i already know the basics.

Thanks in advance for your help!!

MG