They're not nearly as popular as they used to be (since the ascension of web-based message boards such as this) although are still very much used especially for scientific areas.
To use them, grab a newsreader (many email clients such as thunderbird include one) and point to your news server. Check with your ISP to see what it is. often it's just nntp.isp.com or news.isp.com.
Enter that, then hit the subscribe button, should bring up a list of newsgroups.
They're not always easy to follow but they do give you an insight into things you just can't find elsewhere.
newsgroups are also known as usenet. Google "groups" searches usenet.