I think you are talking about blogger blogs..widget is something which adds more features to your blog.Some of the widgets available are tag cloud widget,blog translator widget etc..these widgets improve your blog's look and feel and adds some features..Too many widgets can make a blog messy too... You may love to have a look at my blogger widgets
Using Widgets you can change the look of your sidbars , Like there are Links , categories and archive in the sidbar of your blogs , but if your blog theme is widget ready then you can change the order and can bring some more things to your sidebar , like you can bring tags cloud and all the things which a wordpress script support . I hope this will be clear enough ! Regards, MicroBrain!
They have a variety of uses, easy to move around and for example you could place adsense code into a text widget in your sidebar.