I think they do both of these to show affection and to let you know they're into you. They're both flirty, but not too much for most girls. When they have their arm around you, it's like they're saying, "yep she's mine." and by sitting on their lap, you are closest to them as possible. This is also a clue when the guy STOPS doing it. That may mean he isn't as close to you as he once was. :(
My dear young lady, if you don't know the answer to this question then you should not be sitting on boys' laps just yet. If you were my own daughter, one of the things I would tell you is that NO ONE has the right to do anything to you that makes you feel uncomfortable!!! YOU have the right to say who touches you and how. If you don't want to sit on someone's lap. DON'T And don't worry for a second if the boy gets mad or something... that's HIS problem! If he gets mad, it means he's a jerk and not worthy of you. Walk away. You stand up for YOU. And you know what? That boy and other people too will RESPECT you more for being a strong person! If you're a wimp, people will often take advantage of you and they'll never respect you.
Respect yourself and stand up for yourself and know that you don't have to do anything you don't want to do. If something makes you uncomfortable, that's your cue to say STOP (say it firmly so they take you seriously) and get up and leave if you have to.
What really concerns me is that this is the kind of question you should be asking your Mother, or Father, not strangers on the Internet. Go ask one of your parents, or an aunt, or your Grandma, or some adult you know and trust. These are the people who are going to give you the best answers to life's questions because they love you and want the best for you. There are a lot of things you need to know about boys and life and the world. Ask an adult that you trust.