It's not dumb, but it is complicated. Each relationship in these subsets are going to vary depending on the people involved and their own personal boundaries so it's really difficult to lay out all of the possibilities in one succint explanation.
This is the way *I* define those relationships:
Polyamory: Loving more than one person and having a committed sexual relationship with them. As in, you can be in a relationship with two or more people at the same time and loving each person equally, but differently. The level of committment is the same for each relationship---you'd go to the end of the Earth for their happiness.
Polygamy: Is the practice of one man taking on multiple wives and developing their own intricate family where each wife shares a responsibility for the husband and any offspring in the relationship, regardless of who the biological mother is.
Swinging: Married couples/committed long term couples occasionally bring home or seek out another couple to either hard swap or soft swap. Soft swapping is where the women have sex with each other (or the men) and the other partners watch and encourage the activity while getting themselves off. Hard swapping is where you essentially swap partners for the night. There are no strings attached and no feelings of love, just lust.
Open relationships: One or both people in the relationship seek other partners to date or fool around with but their respective long-term partner is not involved in the relationship or the activity. And this is where things get super complicated because that really varies on the people involved. For instance, one couple may want to meet the prospective partner and get to know them where in another relationship it doesn't matter so long as there is safe sex and honest communication between the people in the committed relationship. Say that Bob and Sally are married and Sally wants to date outside of the relationship. Bob would then say, okay use condoms until [x] number of STI tests come back clean. And there are most likely rules (boundaries) about declaring any romantic or loving feelings that may be developing toward the third person.
Then you have those cases where Bob and Sally are married and they want a girlfriend or a boyfriend and it becomes more of a triad where everyone is involved in the daily activities of the household, share income, share resources and any children that arise from the union.
That's just the very tip of the iceberg, but I hope it helps you understand it a little bit better. Keep asking questions, there are tons of us here that can help you understand. This is just the cliff notes version.