The Apple Worldwide Developers Conference is one of the biggest events for appleOS software developers in every year.
The first WWDC was a long time ago, back in 1998. You can still watch the keynote presentation of every single event since that, but for us developers it's more interesting that there are developer session videos released for everyone.
There is a transcript of WWDC session videos since 2010 available at asciiwwdc.com. They have a search option, if you are looking for something, you can find it quickly.
You can also watch session videos natively from your mac with the help of the unofficial WWDC macOS app, made by Guilherme Rambo. It's an open source application written in Swift, you can download the source from github.
It's always exciting to wait for the next big announcement, like the introduction of the Swift language or a new product. WWDC is a one week event usually in the beginning of June, there are thousands of Apple engineers with lots of sessions and hands-on labs. The keynote is usually live-streamed for the public.
If you want to attend the Worldwide Developers Conference you have to play a lottery. Only 5000 attendees are allowed to visit the convention center, but there are many options for those ones, who misses the opportunity. AltConf is the biggest one.