First off, I wouldn't lose any sleep over it. I imagine you could swap the two in almost any context and I doubt anyone (regardless of origin) would notice.
That said, for all meanings not concerning computers, "disc" is probably more common in British English, and "disk" in American English (that's the OED's take, and matches my experience), but use whichever you prefer. You might think disc would be older, since the word derives from the Latin discus, but there are some pretty old "disk" quotes listed in the OED (one from the Philosophical Transactions of the Royal Society of London, 1665--so no American influence there).
For computers, I have to concede that the Apple page does have a point (though calling it a "distinct difference" is carrying it way too far towards prescription on what's clearly a descriptive issue--for that matter, so is devoting a support page to it). One pretty much always sees "compact disc". "Floppy disks" (for those that remember them) were sometimes called "diskettes"; which term came first, I don't know, but the association between the two probably caused the K spelling to stick. Personally, I would also spell "hard disk" with a K as well--since the term originated by analogy to "floppy disk", that makes sense. I don't think the optical/magnetic split between disc/disk is based on anything like logic, but it does seem to be the way things have developed (mostly: I'd still call a CD drive a disk drive). The OED even acknowledges this: "Some writers have used the spelling disk for magnetic ones (‘hard disk’, ‘floppy disk’), and disc for optical ones (‘compact disc’, ‘laser disc’).". But then, it also lists "compact disc (also disk)".
There might be some technical applications where one is used over the other, but it'll never be based on any logic, since the two words mean exactly the same thing, so if you run into that, just look around to see which is more common. ("Disc" brakes gets the wikipedia entry, but no one would wonder for a second what "disk brakes" were.)
As to why both exist: who knows? Both spellings have been around a long time, so it is surprising neither has died out (since there never seems to have been a difference in meaning between them, at least until Apple got involved). If I were to engage in rank speculation, I would wonder if "disc" didn't come into the language directly from Latin, while "disk" meandered in via the French disque.
Your question makes some wrong assumptions.
These would all be idiomatic:
How was your|the trip? your journey, the things that happened to you along the way
How was the|your travel? the conditions of the transport itself, the roads, the trains, etc
Tell me about your travels. places you've been and things you've seen throughout your life or on an extended trip
Your travel will be fully reimbursed. the costs of traveling
Best Answer
These three words can be synonyms, but have slightly different connotations.
First, travel is usually a verb...
...but not always. In common speech (at least as far as I am familiar), when used as a noun, it is used in its plural form:
Compared to the substantially equivalent sentences:
One could also ask How was your travel?, but it this would have a more specific meaning, for instance, "How was your flight from New York to L.A.?" as opposed to "How was your entire journey, and the time you spent in L.A.?"
Trip and journey are more closely interchangeable, and vary mostly in duration, distance, and formality.
A trip can be a short journey. One can take a trip to the store, but it would be unusual (except in poetic exaggeration) to take a journey to the store
A journey would often imply a longer (in terms of time and/or distance) trip, perhaps to multiple destinations, or with a greater sense of unknown. A journey may not be fully planned out ahead of time.
A business conference to Seattle would probably be described as a trip, whereas a family vacation road-trip from Nebraska, through the Colorado Rockies, camping in Nevada, then stopping in Las Vegas and returning through Oklahoma and Kansas, might be described as a journey.
Often, in colloquial English (at least in the U.S.), trip is far more commonly used than journey, even when describing long/epic travels.
Both trip and journey can also be verbs, but when used as verbs they are not interchangeable. To journey is to engage in the act of journeying:
However to trip is to cause someone to stumble or lose their balance.
To trip up has the additional connotation of causing someone to blunder:
There are also some additional cases where trip and journey cannot be interchanged in some common expressions: