Difference between these selectors [duplicate]

Answer 1

div.demo

will select div with class demo whereas

div .demo

will select any descendant of a div with class demo. Same concept for the second case.

Answer 2

Yes, there is.

div.demo will match a div with class demo

<div class="demo">

div .demo will anything with class demo inside any div:

<div>
  <span class="demo">
</div>

Same with the id selector #.