Circles are the feature that connects you to other people. You can add them to your Friends Circle, Family Circle, Acquaintances Circle or if the other person doesn't fit on any of the above, you can always just follow them with your Following Circle. But it doesn't end there. You can create your own custom circles, like for example, I've created the Web Developers on Google+ circle to add people I know that are Web Developers. So circles is like your Facebook friends, your twitter followers and so much more. +1
The stream is your ol'classic "Wall". A standard feature on all social networks. Watch and read streams of your circles and reply, +1 or re-share posts. You can share on your own stream "status" updates, ideas, links, photos, videos, anything you can think of. You can even filter who will be able to see those streams.
GooglePlus chat the intergraded chat platform of Google Talk. Chat with your circles, start video or voice conversations. Invite others for Group Chat. You can chat to people using Google+ even if you don't have a Google+ account yet, through Google Talk.
Hangouts is GooglePlus' video streaming feature! You can stream through your webcam on GooglePlus Hangouts and let your Circle watch you do what ever it is you want to do... +1
As far as I can tell, Sparks appears to be a search engine withing GooglePlus. You add your interests and sparks searches for content that will interest you.
It's the Android application of GooglePlus. It lets you chat, post images, post streams and read all the content from GooglePlus on your Android enabled device. And it is really fast!
You classic photo albums. You may find pictures you've uploaded on your blogs and other sources there and upload new photos.
GooglePlus offers also a video sharing option for you to upload videos on GooglePlus.
My first impressions of Google's new social platform are great. I like the filtering of your circles a lot and I like it because you can create your custom circles and maybe share something only with a particular group of people in any of your circles. You can have a publicly visible on the web profile or customize what others may or may not see on your profile.
So there your have it, the beta features of GooglePlus.
(I might spare a few invites for anyone interested)