Answer :

Things that use Photosynthesis use carbon dioxide and release oxygen (like plants)
Things that use cellular respiration use oxygen and release carbon dioxide (like humans)

Photosynthesis is the process by which plants take in (use) Carbon-dioxide and water to make food (starch). 
[tex]CO_{2} + H_{2} 0 +sunlight ----> C_{6}H_{12}O_{6}+O_{2}[/tex]
Cellular respiration is the process in which cells use oxygen to break down starch into energy molecules and give out Carbon-dioxide.
                                Cellular respiration:
 [tex]C_{6}H_{12}O_{6}+O_{2}----->CO_{2} + H_{2} 0 +energy[/tex]

So, in Photosynthesis, carbon-dioxide is taken in (used) and in cellular respiration, carbon-dioxide is thrown out (waste product)