For many writers, the goal is for their work to be read by other people. For that to happen, outside of thrusting a copy into the hands of your friends, partner, and family, it needs to be published in some shape or form. There are many ways you can go about this, each with its own benefits and drawbacks.
Trade publishing
Trade publishing is colloquially known as traditional publishing. This is likely the first thing that comes to mind when you think of a published book. If you are published by a traditional trade publisher, you are likely with the Big 5 publisher or another large publishing house. As a general rule, houses like this do not accept unsolicited manuscripts. You will need to write a query letter and submit that to agents. Once agented, they will make submissions to editors within these houses on your behalf as well as negotiate your write and manage your contract. You do not pay your agent of the publisher. In fact, you get paid an advance and royalties. All the financial risk lies with the publisher.
Of course, there are exceptions to the rule. Some large publishers, such as Bloomsbury, and some imprints of Big 5 publishers accept unsolicited manuscripts. You should do your research carefully.
Small presses
Small presses are similar to large ones. You do not pay them. You get paid royalties. As they are not printing large runs, there is less financial risk and it lies with the publisher. In some cases, you may get paid an advance but it would likely be a token payment. Contrary to larger presses, you may not be getting your book into shops. Rather, they will be selling your book through online marketplaces such as Amazon and their own channels.
Independent publishing
Independent publishing, or self-publishing, circumvents the need for the lengthy and tiring submissions process to agents or publishers. You won't get paid an advance as there is no one there to pay you one, but royalties are significantly higher than those offered by publishing houses. Of course, you won't have the specialist support provided by a publishing house. However, you will be in total control of how much money you spend, what you spend it on, and who is working on your manuscript. Small presses tend to have a tight focus on a particular genre or style. They can be a great option for those who want to work with likeminded individuals. It will also be really satisfying to have been the one to work on your work from storyboarding, to writing, to creating a book.
For independent publishing, you will likely need to pay a cover designer and an editor at a minimum. You could also invest in marketing, typesetting, and printing. Publishing through sites like Amazon negates the need to pay for printing but could lead to other limitations. Since there is no publisher, all of the financial risks lie with you. It would be ill-advised to spend money on services if you are uncertain you will earn it back in sales unless you are comfortable investing money as a hobby rather than a viable business venture.
Hybrid or assisted publishing
These types of publishers are also known by the derogatory term, vanity publishers. There is much debate about these types of publishers online. Any criticism is to be prefaced with the fact that some people have used hybrid publishers and have been happy with the result. For a set fee, your book will be edited and a cover designed. With some contracts, you get a set budget for marketing.
Hybrid publishers occasionally offer traditional contracts but more often provide a hybrid contract in which you are to pay a portion of the production costs (often cited as a third). You may then be offered additional packages where you can pay more money for other services. You have little control over who is working on your book and if you are unsatisfied it can be difficult to escalate the issue to get it rectified. You also may not earn out the initial cost of signing with the hybrid publisher. Hybrid publishers advise you that the financial risk is shared, that if your book doesn't sell well both parties lose out. However, in some cases, hybrid publishers have already mitigated this risk by selling enough additional service packages to cover any losses. You run the risk of being the product rather than creating a product. As ever, do your research. A simple web search is enough to find reviews and anecdotes about publishers and editors.
Other forms
Since publishing is simply making information, your work, available to the general public, there are so many other ways to publish your work. If you aren't interested in the above avenues or you want to get creative with it there are so many other ways to be published.
There are hundreds of literary magazines. Getting published in those can build you a great network of writers, people working in the industry, and readers. It is also incredibly fun and rewarding. In some cases, you may get a token payment for your work.
You can create an author website and publish it there. This can be really good for poetry and short story collections. It is a great way to practice marketing and build a following of devoted readers. This allows you to be flexible and creative with the form as well which could feed into your writing style and provide new challenges for world building.
Some local writing groups publish collections of their community's work. Being part of a local writing group has so many benefits other than being published so it is well worth it. And if they do publish a collection it is all the more rewarding to be contributing to building your community's cultural history.
With so many ways to get your work out there, nothing is stopping you. Next, we will unpack more about agents and the submissions process.