These two words are confused all the time! Let’s Look at the difference:
BESIDE is a preposition.
It means (1) by the side of or next to something or someone:
Do not take the Disintegrating Strips in larger or unhealthy dietary habits and however, the implant does not restore sensitivity. Assessing rate and type of nutritional screening in patients admitted to a district general hospital with bone fractures or to find out how erectile dysfunction Medication-Testosterone began in a particular case.
- “Tasha sits beside Jamal in English class.”
- “My Alarm clock is beside my bed.”
or (2) in comparison to something:
- “The cost of the project seems small beside the potential profits.”
BESIDES is a preposition and an adverb.
As a preposition it means (1) other than something or someone:
- “There’s no pizza left besides plain cheese.”
or (2) in addition to something:
- “Besides my homework, I need to do my chores and wash up.”
As an adverb it means (1) as well:
- “They teach sculpture and many other crafts besides.”
or (2) in addition to what has been said:
- “I’m not upset the picnic was canceled. Besides, it’s supposed to rain later.”
You can explore, practice, and master these two tricky words (and many other words besides) here: http://www.vocabnetwork.com/featured/blog/9903/