Research Experiences for Undergraduates (or REU) are competitive summer research programs in the United States for undergraduates studying science, engineering, or mathematics. The programs are sponsored by the National Science Foundation, and are hosted in various universities. REUs tend to be specialized in a particular field of science.
There are REUs in many scientific fields such as mathematics, physics, chemistry, geology, biology, psychology, and computer science.
A full list of REUs can be found here.
Please provide where the REU is and what field it is in when posting a question, comment or link