Hybrid: These tomatoes are made from crossing two varieties which have characteristics that you want to save in their offspring such as earlier maturity, better yield, improved flavor and disease resistance. Good examples are Juliet, a beautiful, little red grape shaped tomato and Sungold, probably the best cherry tomato that will ever pass your lips. (The tomatoes in the cover picture at the top of my blog are Sungold). Early Girl, Celebrity and Carmello are other hybrids. An F-1 Hybrid means it is the first generation between two different parents.
While you can save seed from it, be advised that it will not come back as the same thing you grew the first year.
Sometimes folks ask me if hybrids are GMO. They are not and I will devote a post on it later.
Open Pollinated: These are tomatoes that have the same parents as themselves. You can save seed and they will come true. All heirlooms are OP and have been saved for generations because of superior qualities like adaptability to a local area and flavor. Heirlooms are another post also.