Let's start with a blurb. Most of us know that the blurb is the text on the back cover of a book. It can often be a decision-maker for readers, as it gives them a brief description (normally about three or four paragraphs) about what lies within the book they are thinking of buying. Crucially, the blurb does not give away too much; it is designed to draw the prospective reader in and persuade them to buy the book. It promises them a great read, with twists and turns, but does not reveal the ending.
A synopsis, on the other hand, goes into a lot more detail. A good way to think of a synopsis is to think of it as a detailed timeline, explaining the most crucial events of the novel, including the ending. One major difference between a blurb and a synopsis is the style. Whilst both are commonly written in present tense, a synopsis is often written in a more formal way, and unlike the blurb, does not contain techniques such as rhetorical questions.
A good way to remember the difference between a blurb and a synopsis is to look at the last couple of letters in the word 'synopsis'. Notice the word 'is'? The synopsis is saying this is what will happen in the novel.
Both blurbs and synopses are used by authors to sell their work, but to different audiences. Whether you decide to self-publish or go with a publisher, both of these need to say the right thing about you and your novel. Needless to say, they both need to be well-written and proofed, but it is critical that you spend time on both of them and do not rush them. It can be difficult to condense 70 000+ words into a page, so start by noting down the big climaxes in your novel and use a paragraph for each. Careful planning will prove vital in the long run.
If you need help with writing blurbs and synopses, do not be afraid to ask! Friends and even authors and editors are often more than willing to lend you their advice.
Best of luck, writers!