An open college is usually open admissions. That means that so long as you have a HS diploma or GED, you will get in. They let in everyone who applies, basically.
A selective college gets so many applicants that they have to screen out those who don't meet certain standards, so for a selective college, you'd need to be in a certain GPA level and/or have an SAT or ACT score that falls into a certain range to get in. They admit only a certain percentage of those who apply.
