Ah, well. 

Although, people talk about making things simple for themselves but trust me they make it more complicated than easy. 

A person can pull up to you and ask if you are in a relationship and if they say yes this would be the for sure conversation:

Me: Hey, yes, I am in a relationship with this girl. I really like her.
Him: Are you sure?
Me: What do you mean?
Him: Are you in a relationship, or are you just dating her?
Me: I don't understand the difference.

But don’t worry I think I got you covered in understanding what these two terms mean.

So basically, 

Being in a relationship relies on giving and receiving a mutual full-blown commitment to a person you spend quality time and romantic times with.

One of the major differences between dating and relationships is having your own idea about what the two are. While relationships are based on mutual agreement and trust, dating isn't the same. 

For some, casual dating is not mutual. The idea of exclusivity while with someone may differ. Some people like to exclusively date each other while others like to date other people and don't want to be committed to just one person.

This one's a given. Commitment is the key in a relationship, that's why it's a relationship. People commit to stay together and look forward to a future together while dating (mostly) lacks commitment of any sort. The only commitment people may have while they're dating is to get an ice cream together, after dinner!

Communication differs largely in a relationship vs while dating. While you're in a relationship you'll be communicating with your partner often, about every little thing. Be it telling them about the small little details from your past or asking them to snore a little less while they're sleeping! Dating is different. Like deciding where to eat next or a casual banter is something couples who're dating usually indulge in.

While both dating and relationships are meaningful, one prioritises less than the other. When you're dating someone you're not necessarily serious about them so you prioritise other things along with them like work, friends, activities etc. When you're in a relationship, your dynamic with other people might change entirely because the person you're with takes prime importance in your life. They come before family, friends and even work.

Of course expectations vary when you're dating someone or when you're in a relationship. You expect less from the person if you're dating them. It's normal for one person to be more into the dynamic than the other person but because you both know it's casual, there are no expectations of the future etc. with them. A relationship, however, is built on expectations. You expect every little thing from your partner. From patience to even a bright shining future, it's all on autopilot.

Hope it gave you a little clarity over self-made doubt and differentiation between Dating and in a relationship.