What Is a Valid Argument?

 
As part of the basic concept series, Janet Stemwedel explains arguments [Basic concepts: arguments]. For example, she says,

Here's an example of a valid argument:
1. Britney Spears is from Mars. (premise)
2. Martians have astounding vocal range and are great dancers. (premise)
3. Hence, Britney Spears has astounding vocal range and is a great dancer. (conclusion)

Are you convinced that this is a valid argument?