Partial view .ascx

Question

I am adding a view for a controller method, I am getting one checkbox "Create a partial view (.ascx). now my questions are : [If possible provide me the link]

1. What is this partial view ? Why should I use this?

2. If this for reusability, please give me an example that I can show same error page for different controller with different text in it.

3. Is it a good practice to use partial view? and how can I take benifits from it.

Thanks in advance.