Ok I'm a new to this road bike thing so pls indulge me, what are the differences bet all this categories? what speed do Cat 1 guys usually do??and how can one know what category is??Just a question from a newbie..

It really depends on the course, a massive climbing course will be much slower than a flat or downhill course. ive heard cat5(the lowest) ride about 25mph on average on a flat-ish course. but also, this is made much easier by drafting, where youre not fighting the wind constantly. get out and get used to your bike, then as you get stronger(or even from the beginning), try group rides. just google road bike group rides in your area. group rides will get you used to racing style riding on the more advanced rides. ive also heard it takes 3-5 years to become a decent racer, but that doesnt mean you cant start earlier than that, just dont let the faster people discourage you. and dont enter a race before trying group rides. i know i would feel REALLY bad if i caused a wreck because im new. hence, no races yet. Im still trying to hit up a few group rides, but my work schedule conflicts with anything but mountain bike or casual group rides

Everyone starts at cat-5. After doing 10 races they become cat-4. After that it is a matter of collecting points based on how you place in races to advance through the categories. A cat-1 generally very fast. How fast a race moves depends on the dynamics of the race. It doesn’t really matter how fast you can do a solo ride, if you haven’t raced before, you’re a cat=5.

It's really not about speed, but about distance or time of a race. Higher category riders ride longer distances. So, for example if you are a 4/5 and are riding for 45 minutes (many of the races in the US are timed because it's not unusual to only have use of the race location for a certain period of time), a 1 race might be 90 minutes. But their speeds could be pretty close to the same.

Lower cat races tend to be hammer fests where the higher cat races are a bit more strategic in nature due to the longer distance and almost all the riders will be attached to a team. Low cats can have a few to many unattached riders. This is because bike teams tend to be clubby in nature. They can be hard to join, which is too bad.

So one advances in categories base on how many times you win races? Who detemines if you are ready to go the next category?can one skip from cat 4 to cat 1 ??

Most of your answers can be found at the USAC site, in the rule book: https://www.usacycling.org/news/user/story.php?id=4220

Specifically: https://www.usacycling.org/forms/USAC_rulebook-3.pdf