IQDB is a tool where one can upload an image and use the website to find a post on the image among an array of "booru" databases. It is primarily used for finding a source of an image. Results may have images unsuitable for children, though.
Very useful tool for the otaku people desperately looking for a better version or a source version of an anime picture. It browses the databases of many boorus. But this is also IQDB's weak point: boorus contain a high number of pornographic material, sometimes of surprising nature! So this is definitely nor for the newbies, neither for the children.
IQDB can be compared to SauceNAO which browses through Pixiv.
A way to look for source images on the Internet, directed to wider audiences, is Google Images's reverse search tool.