Fitting in is about conforming to others' standards in order to be accepted.  Some examples of ways to fit in:

• Losing weight
• Wearing current fashion trends
• Listening to the same music as your peers
• Agreeing with the opinions of the group you desire to fit in (and not saying so if you don't agree)
• Blending in to the crowd, not making noise
• Gossiping about or judging "outsiders"

Belonging is about being accepted for who you truly are.  Some examples of ways to belong:

• Unapologetically being your true self
• Accepting your body's natural size/shape and celebrating it with your clothing
• Having your own opinions
• Sharing similar values to the group which you belong
• True relational connections with others in the group
• Letting down your guard and fully participating

Personally, I'd rather belong than fit in to a group – whether it be friends, family, coworkers, or a club.  Fitting in is a lot of work because it often contradicts who you naturally are.  It can feel like a charade.  Belonging feels good because you don't have to work to be something you're not.  The energy you put in to the dynamic is reciprocal.  When you're trying to fit in, there's little room for error.  One misstep and you're an outsider.  But when you belong, mistakes are allowed and often result in deepening of relationships as they are repaired. 

In essence, fitting in is driven by shame, and belonging is driven by connection.