While science fiction as a whole generally refers to fiction told through a film of scientific advancement, the stories themselves can take on any number of themes or tropes to make a point or tell a story.
This is just a short list of some of the sub-genres within SF and some examples. This is not meant to be an exhaustive list or a "best of", just some of examples that well-represent a certain sub-genre.
A sub-genre typified by scientific accuracy and logic, or as close to it as can be achieved within the context of the book's fiction.
This sub-genre, as opposed to hard science fiction, uses technology and science as storytelling vehicles to progress plot, character relations, or other areas of academic discussion (such as psychology, sociology, anthropology, and political science).
This sub-genre is most clearly focused with it setting in the apocalypse or post-apocalypse. There is no consistency in themes seen, other than the questions that follow an apocalypse or post-apocalypse.
Pretty self-explanatory, this sub-genre uses science fiction tropes to tell military stories. Tale about invasions, PTSD, military technology, and more are present here. Themes will typically involve some kind of military story.
This sub-genre uses science, technology, and a lot of imagination to tell a grand, romanticized tale across an expansive and exotic setting, with larger-than-life characters.
This sub-genre examines the polarized extremes of what our contemporary society might develop into if corporate greed is left unchecked. The settling is typically in the near future, and references contemporary issues plaguing modern populations. Expect tropes like unchecked scientific development and rapidly degrading standards of living!
This list is far, far from a complete list of science fiction genres and the examples within. These are just some starting points and examples to help you start your reading journey. The examples listed may not be the most popular examples within each sub-genre, but in my experience they were engaging, entertaining, and thought-provoking entries that left me wanting more.