btn class of Twitter-bootstrap can be used with any of the following html elements.

1. anchor
2. button
3. input with both type="button" and type="submit"

Below are examples of all possible use cases of btn class

<a class="btn" href="#" role="button">Link</a>
<button class="btn" type="submit">Button</button>
<input class="btn" type="button" value="Input">
<input class="btn" type="submit" value="Submit">

Although btn class can used in any of the above four ways but it is highly recommend using the <button> element whenever possible

Image of the above code is attached below

Source

twitter-bootstrap-3 has provided four different sizes of buttons

1. Large button btn-lg
2. Default button does not require any btn size
3. Small button btn-sm
4. Extra small button btn-xs

 <button type="button" class="btn  btn-lg">Large button</button>
 <button type="button" class="btn">Default button</button>
 <button type="button" class="btn  btn-sm">Small button</button>
 <button type="button" class="btn  btn-xs">Extra small button</button>

Image of the above code is attached below

Source

Glyphicons can be used in text, buttons, toolbars, navigation, forms, etc (Source: W3Schools). Glyphicons are basically icon forms that can be used to style any of the aforementioned. These examples outline the usage of glyphicons inside two types of buttons by simply using a span inside the buttons which have a class of type glyphicon:

HTML Button

<button type="button" class="btn btn-info">
    <span class="glyphicon glyphicon-search"></span> Search
</button>

ASP Button

<asp:LinkButton runat="server" CssClass="btn btn-info" >
    <span class="glyphicon glyphicon-envelope"></span> Email
</asp:LinkButton>