Wort

Wort is what you have before fermentation starts: sugary liquid extracted from malted grain during the mash, or dissolved from malt extract in hot water, then boiled with hops. It carries the fermentable sugars, proteins, and color that will define the finished beer, but it contains no alcohol yet. Everything you do up to pitching yeast is really about building good wort.

The boil is what turns raw sweet liquid into stable wort. Boiling sanitizes it, drives off unwanted compounds like DMS, helps proteins clump and drop out, and lets you add hops at set times for bitterness, flavor, and aroma. Once the boil ends, wort needs to be cooled quickly, since warm sugary liquid sitting in open air is an easy target for wild yeast and bacteria.

New brewers sometimes treat wort like it is already beer and get careless with sanitation at this stage, which is backwards. Wort is actually at its most vulnerable right after the boil, once the heat that was protecting it is gone. Cool it fast with a wort chiller, get it into a sanitized fermenter, and check the starting density with a hydrometer or refractometer so you have an original gravity reading to compare against later.