This is really going to depend on the schools’ curriculum. Psychology and biology are the most common majors, followed by neuroscience. Neuropsychology and neurobiology are generally not offered at the undergraduate level; they’re usually graduate specializations. A few colleges may have them.
If you plan to become a physician, it honestly doesn’t matter which one of these you pick. You could major in art history and still become a neurologist, because all of the relevant information for that career you will learn in med school and in your residency afterwards. So yes, you would be totally fine to go to Maryland and major in biology or psychology rather than going to Pittsburgh and paying a lot of extra money for a more specialized major. (Also, UMD has a physiology and neurobiology specialization within its biology major: http://biology.umd.edu/physiology-and-neurobiology-specialization-phnb.html)
To answer your original question:
Psychology is the study of mind, brain, and behavior. Your classes will focus on the ways in which human cognition, emotion, learning, and social behavior work and - to a limited extent - some biological structures that make these things possible. You will also take some classwork in psychological research methods and a class in statistics. I was a psychology major in undergrad, and generally speaking, psychology coursework is pretty easy on the undergrad level.
Biology is the study of life and living organisms. Your classwork will focus on the structure, growth, evolution, function, and distribution of living things - how they work mechanically and chemically, how they are grouped together (taxonomy), how they live in harmony (or not), their life cycles, the structures that make them up (all the way down to the cell), and how they have evolved over time. At some schools you will take a semester or two of calculus (sometimes ‘calculus for life sciences/biology majors’), and most colleges will require a research methods course or integrate the research methods into laboratory courses in the major. I have heard variable things about the difficulty of biology classes - I think that the difficulty seems to come with the sheer amount of stuff you have to memorize. Also, in some schools the lower-level biology classes are difficult because they serve as weed-out classes for the pre-med kids (or, conversely, you have hypercompetitive pre-med students who are breaking the curve). Once you get to upper-division classes they thin out.
Neuroscience is kind of a combination of the two, but goes deeper. Simply put, neuroscience is the study of the nervous system, including the brain. As a field, neuroscience is based primarily on biology and psychology but also borrows a lot from other fields (chemistry, physics, linguistics, sometimes math, sometimes computer science). Neuroscientists study the structure and function of the nervous system as well as its outcomes in behavior, cognition, and emotion in humans (and often non-human primates). As a neuroscience major, you’ll probably take courses in a lot of fields; what those courses are depends on how well developed the neuroscience major is. At some schools, the neuroscience major is little more than a compressed double major in biology and psychology. At other schools, neuroscience is its own department, with professors dedicated to it and classwork that is developed specifically for the major that blends the bio and psych together with other disciplines into actual, self-contained field-specific classes. I’d wager most schools fall in the middle - they may have a core of some neuroscience coursework that was developed specifically for the major but many of the electives and cognate courses are in the biology, psychology, and perhaps chemistry departments.
Neurobiology is simply a subfield of neuroscience that focuses primarily on the biological structure of the nervous system, and sometimes how it interacts with other biological structures within the body. Neurobiologists focus primarily or solely on the biology part of neuroscience. Neuropsychology is both a subfield of neuroscience and a subfield of psychology; it focuses on the structures and functions of the brain and how they relate to psychological processes like thinking, feeling, and behaving.