Read the first two pages in order. They define a component and a collection, the whitelist that decides what an author may add, and the one loop every theme writes, which the rest of this guide refers back to. The nesting page then covers blocks inside blocks and what depth costs.
The last two pages turn to blocks that are not authored on each page: global components, which are shared and reached by identifier from any template, and computed blocks, which list records with the query helper instead of holding content. Widgets, which older sites used for computed blocks, are deprecated.