This one is going to be super quick because, while the terminology can be confusing, the differences among these the three are pretty clear.
Loglines
Loglines are simply a 1-2 sentence summary that encapsulates the set-up of the movie and makes the script reader (usually a member of the industry) want to read more; while there are many different formulae to express them, your typical logline will usually include the protagonist, the obstacle he/she needs to overcome, the stakes, and the hook.
All the logline does is give the reader a taste of what’s to come and (hopefully) pushes the script to the top of the reading pile.
Let’s look at an example from Seven
Logline: Two detectives, a rookie and a veteran, hunt a serial killer who uses the seven deadly sins as his motives.
We have a protagonist (or two here): a rookie and veteran detective.
We also have the difficult task ahead: hunting a serial killer.
We also have stakes because, with a serial killer, they’re implied: precent an increased body count.
We also have a hook: he’s using “the seven deadly sins as his motives.” This device makes him different from all the other serial killers on the loose.
Taglines
A tagline is the phrase intended to pique the interest of the audience so that they’ll buy tickets to the movie. (Think about advertising – just as every product has a tagline (i.e., “America Runs on Dunkin,’ so does every film).
Let’s Look at Seven again
Tagline: Seven deadly sins. Seven ways to die.
Note how the tagline is succinct (seven words); it piques your interest (what? there are seven ways to die?); it leaves room for curiosity (who dies? how?); but it doesn’t mention the protagonists, the serial killer, or anything else.
The intentions of both a logline and tagline are to pique interest, but unlike a logline (whose intended audience is a smaller industry audience), a tagline has a mass audience. When it comes to piquing interest, the larger the audience, the less should be said.
A screenwriter needs to come up with a logline; as for generating a tagline, that would most likely be the producer’s or studio’s job.
Sluglines
Sluglines exist within the script, and they denote certain logistics of the scene (interior or exterior, location, and time of day).
Sluglines are intended for production, as they tell everyone on the team what is needed. These details strongly influence shooting schedule, location choice, and anything else pertaining to that.
And we’re back to Seven
As you can see, the slug of “EXT. COUNTRY CHURCH — DAY” appears at the top and introduces the action lines below. Notice that the EXT. denotes “Exterior,” there isn’t a lot of description for the location (“COUNTRY CHURCH”), and “DAY,” of course, indicates that the production team will shoot during the day.
Now, there are such things as subslugs; as in, if you’re at one location (i.e., country church) and then have a bunch of different sublocations (i.e., church lobby, aisle, etc); but since there a bunch of different ways you could format them, the topic deserves a completely different blog post.