The main difference between dating and being in a relationship is the level of commitment and usually an honest and open conversation with your partner. Dating can be an adventure involving casual sex and fun, but a relationship is a more serious and romantic affair. Dating doesn't involve exclusivity, but a relationship is based on loyalty. There's more love than lust in a relationship and being your “stupid, careless self” is OK.
Dating is when two people are interested in getting to know each other but don't have a serious attachment or commitment to the other person. They choose to refrain from planning for the future and agree that the relationship will continue informally. It may be exclusive or non-exclusive, but they haven't yet entered a committed stage. The relationship is light and informal, without conditions. The difference here is that you choose to date someone exclusively by investing more energy and time with that person.
Early in your dating phase, there are no conversations about “us” because you don't plan to build a future together with the person you're dating. Some people may date solely for sex, while others may date with the goal of finding a serious and committed relationship. When do you know that this is all and that now there is nowhere to go but to move up in this relationship? Many even subscribe to the 3-date rule, which basically makes you decide if it's time to sleep together or not. Dating exclusively means that you and your partner have talked about your future and have agreed to go out just for the time being. It starts with the online texting stage, the first date and then deciding if one wants to continue with this or not. You may not know the person well enough after a month and a date or two, or you may feel like you know them well very quickly. But how do you truly love someone in a relationship? A relationship is rather based on sacrifices and commitments.
Dating exclusively means that the two of you have decided to take your relationship one step further and are agreeing to only go out for a while. They may decide to be sexually exclusive, but still date other people, be sexually open but only date one another, or enter into a completely exclusive relationship in which the physical and emotional aspects of the developing relationship are reserved for each other.