In vitro Fertilization, or IVF is a technique of creating life by fertilizing an egg outside the human body, thus creating a zygote. In the US, in vitro fertilization can only be performed for the express purpose of achieving pregnancy, wherein the zygote is allowed to grow for several days in vitro, thus becoming an embryo, which is implanted in a woman (typically the mother, but sometimes a surrogate is used). This procedure is used when traditional pregnancy cannot be achieved.
This technique has the potential to benefit families by blessing them with children, but it is typically used more often by liberals than conservatives since a large percentage of the new life formed by in vitro fertilization (zygotes) do not progress to the embryo stage (i.e. die) and several embryos must be sacrificed during implantation to ensure that at least one embryo attaches to the uterine wall. Thus, in the typical IVF procedure, many eggs must be fertilized and those embryos that are not implanted are often killed or cryogenically preserved for a short time and later used in future implantations or ultimately destroyed. Instead of killing these embryos, conservatives have created a program for these embryos to be adopted by new parents, sometimes over the objections of liberals and parent's rights groups who believe the parents have the right to do with their embryos as they please (i.e. kill them). 
IVF clinics are the only source of embryos for the creation of new embryonic stem cell lines in the US; embryos at not created ad hoc. In the past, parents could donate their unimplanted embryos for this purpose rather than killing them outright or allowing others to adopt them. However, in nearly all cases the process of harvesting the cells needed to make embryonic stem cell lines results in the death of the embryo.
The Catholic Church is opposed to the IVF process because it separates procreation from sexual intercourse, as well the need to sacrifice many zygotes and embryos in the process.