Your WordPress theme provides most of the information on how your WordPress site will look and function. There is still another layer though that WordPress provides to further customise what appears on your pages. Each WordPress theme usually provides one or more areas set aside within the theme where you can place widgets.
Widgets are basically blocks of additional content that you can add into any of the specially designated spots within the page. WordPress provides you with a selection of pre-built widgets that contain a variety of information about different aspects of your site. For example you can display a calendar, a tag cloud or links to monthly archives of all the content added in those months. You can also create your own widgets containing anything at all using a widget that simply allows you to enter the HTML that you want.
To install a widget you need to go to the Widgets option in the Appearance section of your WordPress admin area. There you will see a number of boxes on the left that are all of the widgets that you currently have available. Just what you have there is going to depend on which theme you are using and what plugins you have installed. Below the available widgets you will find an area for inactive widgets. Inactive widgets are those that you have customised but which you are not currently using in your pages.
To the right You will see a list of those areas that your theme provides that give you one or more spots within the pages to display widgets in. Depending on the theme there may be only one widget area or there might be more than a dozen different widget areas to choose from.
Installing a widget into your pages involves two steps. The first step is to first open the widget area you want to install a widget to (if it isn't already open) by clicking on the down arrow at the right of the widget bar and then place your mouse button over the widget on the left that you want to install and press the mouse button. Holding the button down you now drag the widget across the screen to the area you wish to place it in. If you already have widgets in that area then you need to move to the spot relative to those already there. When you release the mouse button the widget will be installed into that spot in that widget area.
The second step in installing a widget is to customise this particular installation of the widget. You do this by clicking on the down arrow at the right of the widget you just moved. This will display those fields that you are allowed to customise for this particular widget. There is usually a title field to display a title at the top of the widget with whatever else you need to enter for the particular widget below that. With some widgets there is nothing else you can customise and so only the title can be entered. The text widget will have a text area where you can enter whatever HTML that you need to display whatever content you want. Once you have customised the widget you should press save and then close.
Your customised widgets also have a delete link in them that will allow you to delete the widget if you no longer want it but a better alternative if you no longer want a particular widget displaying is to drag it to the inactive widget area where it will be available to put back should you decide to do so in the future.
If you have more than one widget area in your theme then you can move widgets from one area to another. Simply make sure that both the from and to widget areas are open and then drag the widget from the old area into the new area.
Adding and changing the widgets displayed on your site is perhaps the easiest way to make minor changes to your site layout without having to change the theme.