The initial goal for children coming into foster care is almost always reunification with their biological family. You, as the foster parent, are asked to help support this goal. Children will typically have regular visits with their family. In some states and counties you'll be encouraged to form a relationship with the birth parent(s), either through sending them letters and pictures, helping to supervise the visits, or taking phone calls from the parents.Children in foster care almost always have "issues." They have likely experienced any number of the following: substance exposure in the womb; emotional neglect; physical neglect; physical abuse; sexual abuse; emotional abuse. In some situations, police have come into their home at night, removed them from their beds without a word, and dropped them off with complete strangers. Without any chance to say goodbye, they've left their family, friends, neighbors, teachers, classmates, pets, clothing, toys, and favorite blanket or stuffed animal. In most cases they never had much reason to trust their previous caretakers in the first place, so why would they ever trust you? And that's just the beginning. A lot of people plan to get around all of this by only accepting infants. Don't forget that many infants in foster care have received very poor prenatal care, and were exposed prenatally to drugs and alcohol. You'll also wait a potentially long time for a placement of a single infant. Having said all this, there are also lots of very healthy children in foster care who thrive with proper love, support, and structure. The gamble with foster care is that you don't know what you're going to get. You typically have to say yes or no to a phone call for a possible placement within an hour - often within minutes. The person calling you often has very little information, and sometimes it's incorrect - in our "say yes or no right now" phone call, they were three years off on one of the kid's ages.
If you're serious about pursuing foster care for the purpose of adoption, you'll want to speak to some agencies in your state. Different states have different licensing procedures and categories for straight foster care vs. foster-to-adopt, vs. adoption-only. Different agencies specialize in those different categories, so shop around for an agency that matches your goals. Be honest and upfront about your desire to adopt.
Understand that foster care doesn't always result in adoption. Many foster parents who wish to adopt choose to take "low legal risk" placements. In this case, it is believed that the biological parents' rights will be terminated, and the state is trying to place the child in a pre-adoptive home that will be willing and able to adopt the child as soon as they are legally free for adoption. There are many stories of these cases turning around at the last minute and the children going home.
Be prepared that until the adoption is finalized, nothing is certain in the world of foster care. But also know that even if your ultimate goal of adoption isn't realized with your first (or second, or third, or fourth) placement, you'll have done a world of good for a child who needed a safe and loving home for the time they were in your care.