Archives
A monthly archive of your site’s Posts. If you only have a few Posts then you probably don't want to show this Widget. When you have lots of Posts over a period of time and your Posts are time sensitive then this Widget could be worth adding to your Sidebar. If your content is evergreen then I wouldn't bother with Archives. This widget can be useful if you're writing about a constantly changing subject such as news, the internet, YouTube or WordPress. People could need to know that your content is up-to-date.

Calendar
A calendar of your site’s Posts. Similar to Archives. It could have a detrimental effect on your visitor's willingness to stay and read old, out-of-date Posts.

Categories
A list or dropdown of categories. This can be a useful Widget if and when you assign categories to your Posts. Categories are the broad subjects of your Posts. You could create a category called Reviews and assign to that category all your Posts which are reviews of products and services. On my FreeAndCheapEbooks.com website I have my eBooks categorized as Fiction and Non-Fiction then I also categorize them according to genre. So 'Dracula' would be in both the Fiction and Horror categories.

Next Page: Custom Menu, Meta and Pages Widgets
Page 4: Recent Comments, Recent Posts and RSS Widgets
Page 5: Search, Tag Cloud and Text Widgets