Determinism is quite easy to define, as it's a term stemming from folkbiology notions of essentialism. The idea of determinism is the idea that a person's entire life is predetermined by some sort of factor or set of factors other than their conscious mind. The idea of determinism is that no one actually chooses anything, and instead what we perceive as us making a choice was already predetermined by other factors. Given those other factors, there's no way that a person could have made a different choice. Determinists are limited in their ability to describe certain behaviors and phenomena, however, which is a major limitation of the view. It can also be argued that determinist viewpoints are reductionist.
Free will is a bit harder to describe, but in essence it could be said as the opposite of determinism. If determinism is the notion that people don't truly make decisions, then free will is the notion that people do make decisions. This, of course, does exist to varying degrees depending on who you ask. So rather, it's not that humans do always make decisions, but rather that we always have the capacity for decision making, regardless of whether it's used or not. A common problem with hardline free will defenders is that they can often downplay or even ignore social problems because they see acknowledging certain social problems as giving some credence to determinism, for example some free will defenders may deny that systematic racism exists in certain societies. However, the more nuanced free will defender will acknowledge that social problems exist, but will not tie down these social problems to any specific trait in of itself, but instead to the normative values placed on said traits. This allows them to tackle a problem such as systemic racism without giving credence to determinism.