Chemistry is the study of matter: what it's made of, its properties, and how it changes. 

Matter is the "stuff" that makes up the world. It's anything that has volume and mass. 

MATTER

• Rocks
  
• Dirt
  
• Water
  
• Air
  
• Skin

• Plastic
  
• Paper
  
• Trees
• Metal
• Milk

• Cars
  
• Gasoline
  
• Popcorn
  
• Hamsters
  
• Matches
  

NOT MATTER

• Heat
  
• Light
  
• Energy
  
• Fire
  
• Electricity

• Ideas
  
• Information
  
• Memories
  
• Dreams
  
• Emotions
  

• Sound
  
• Gravity
  
• Time
  
• Motion
  
• Rainbows
  

Volume is how much space something takes up.

Mass is how much matter (or "stuff") something's made of (or is inside something). 
(It's very similar to weight, except weight depends on gravity.)

Density is the relationship between volume and mass.
It's how much "stuff" (matter), fits in a given space.
You can think of it as how tightly stuff is packed into a space. 
It can be calculated with the formula Density = Mass ÷ Volume.