XML Tutorial/Introduction/Attribute

An attribute creates additional information without adding text to the element.

<animal>
  <name language="English">T1</name>
  <name language="Latin">T2</name>
  <project>project1</project>
  <weight>3 points</weight>
</animal>
Attribute names must follow the same rules as for valid element names.
If a value contains double quotes, use single quotes to contain the value (and vice versa).
An attribute may not contain a reference to an external entity.
An attribute may not contain the symbol <, use &lt; to represent it.

Attributes are simple name/value pairs associated with an element

You can"t have more than one attribute with the same name on an element.
Order of attributes is not relevant.
They are attached to the start-tag, but not to the end-tag. 
<name nickname="007"> 
    <first>Bond</first> 
    <last>James</last> 
</name> 
Attributes must have values.
Those values must be in quotes. 
Either single quotes or double quotes are fine, but they have to match. 
<input checked="true"> or <input checked="true">