Early on polls are conducted at the behest of interest groups and candidates for the primary purpose of generating enthusiasm for the candidate or cause in question. These are recognizable by the phrase "adults" used to describe respondents.
Later the polling companies, in an effort to not look like complete idiots, adjust their methodology to edge closer to the actual state of things. Here you see words like "registered voters", although Mickey Mouse and Do-Wop Diddy are registered in more than one district. If these results show your lead shrinking, it's time to panic.
Last stage polls are of "likely voters", myself, for example, which gives the polls significant weight. If I tell a pollster that I will vote for X and against Y, you can count on levers being pulled and/or boxes being checked to that effect. When a pollster begins to use words like "historic", somebody's goose is likely well and truly cooked.
Personally, I'm hoping to see the word "biblical" come into more common use when projecting the upcoming elections, as in "political devastation of biblical proportions".