There are many approaches to handling multiple partners, whether romantic or sexual.
I am going to describe a hierarchy as used by many polyamorists. Some have different definitions or criteria for the levels than I do. Some do not use this "system" at all. This is the one I go by.
PRIMARY: There can ever only be one primary. There may also be no primary, only secondaries and lower. There is never more than one primary at any given time. Primary is the only level that has a limit on number of people the label can be applied to at once. This person is the one you are most concerned with, the one who receives more of your attention and/or feelings. My only criterium for being the primary is that the person is one you are in love with, and if you are in love with more than one person, then it is the one you give the most dedication and energy to.
SECONDARY: There can be any number of secondaries. I consider secondaries to be people that you are in serious relationships with. You may be in love with them or not. It's about the emotional involvement attached.
TERTIARY: These are budding relationships and friends with benefits. There may be no emotional involvement, but almost always there is at least some. This applies to fuckbuddies whom you have no feelings for as well as casual dating. This might also apply to low-key online or long-distance relationships.
QUARTERNARY: One night stands or very short-term relationships without sexual involvement. You might take a quarternary after a night at a bar. You might have one while you're out of town for the weekend and meet someone you're interested in. I personally think that with the out of town example, the label "quarternary" only applies if there was limited sexual involvement; if it involves what you consider to be enough (YOU define "enough"), it qualifies as tertiary.
Other variations exist. You could separate these levels out even further, making two levels out of friends with benefits and casual boyfriends/girlfriends. Secondary could be broken down into two separate tiers, one of people with whom you are in a loving relationship and one of people with whom you are in a serious relationship but do not quite love. Like I said, the hierarchy listed above is the system I use.
Variations on this "system" are popular, though the definitions are very subjective and are always open to the definition of the person who uses them. There are no correct criteria.