I'm talkin' 'bout a potstill here, and these are the terms I learned when younger. Many folks use different terms for the same thing, or stage of the run.
Charge a still with a fully fermented mash or wash. Start the heat to the still and bring it up to operatin' temp. Collect ALL distillate, makin' no cuts. Collect untill the abv% of the distillate drops to 10%. You should have about 1/3 the origional volume now.
You now have done a strippin' run. Huzzah!
Now you have an option: Do yer second run now, or ferment out and strip two more washes or mashes.
If ya do yer second run now ya get X amount of distillate from runnin' the still twice. If ya combine three stripped batches and run 'em the second time all together ya get 3X amount of distillate from runnin' the still four times. Huge savin's in overall time and energy by combinin' stripped runs, but we don't always have that luxury.
The Second Run: Here is where we do "cuts" and divide the run into foreshots, heads, body, and tails.
Charge and fire-up yer still. Adjust heat as to get a fast drip, or a tiny stream from yer condencer (I use a coil)[The stream should actually be tiny... size of a pencil lead or a bit smaller]. The first bit out of the coil should be run into a graduated cylinder, or measurin' cup (with ml graduations). Collect and discard 200 ml, or more, per 5 gallon wash or mash. This is foreshots. Foreshots are where most of the methanol, and bad nastys are.
So if ya combined three stripped 5 gallon rum washes into roughly 5 gallons again, and were discardin' foreshots, you'd wanna collect and discard AT LEAST 600ml. This is a pot still we're talkin' 'bout here.
Now yer collectin' heads. Heads often smell and taste sweetish. Heads are a mix of ethanol and the lighter alcohols. Collect and discard, or save yer heads. But this isn't yer drink... don't drink heads unless ya want a nasty splittin' headache and a sour stomach.
Next comes the body, and the body is what ya age/flavor/water down and bottle. The body is yer drink. Where ya make yer "cut" from head to body is based on the intended drink (whiskey, rum, brandy, etc) and upon the distillers sence of smell and taste. The distillers experience tells him or her where to make the cut.
Collect and save the body.
When the body is finished (up to the distillers experience) make the cut to tails. If this is all new to you and yer a new distiller... I suggest ya make the cut from body to tails when the output of the still drops to 50 or 45% abv.
Tails are usually (but not allways) low proof and watery... the still will start dischargin' the distillate slower and the nice tiny stream will change in appearance to a thicker stream that will eventually break apart into large watery drops. Tails from some stuff, like whiskey mashes, can be oily. Tails are most allways stinky, and become more nasty and stinky as ya get further into the tails.
Save or discard tails. I save 'em. It's probably silly of me, but I save tails 'till I have a full charge for my still, then I run it and mark it "tails XX". When I have a full charge for my still of tails XX I run it and collect till the overall collected amount of distillate is roughly 70% (140 proof) and I use it as fuel in a few alcohol stoves and I use it as a solvent.
A word about cuts: With experience yer nose and tongue will guide you. With enough experience no tastin' will be necessary... a good sniff will be all ya need usually. Also hydrometers are way better gauges of where ya are in a run than thermometers for a potstill.
I hope this was clear enough, and is able to help.
I wish ya luck.