The cat is a small, furry animal with four legs and a tail.
A cat is a small, furry animal with four legs and a tail.
Cats are small, furry animals with four legs and a tail.
A mobile phone is a portable telephone that can make and receive calls.
Mobile phones are portable telephone that can make and receive calls.
The mobile phone etc. same thing.
The donut is a small, circular cake, fried in hot fat, either with a hole in the middle or filled with jam.
Same thing: "A donut is" or "Donuts are".
When talking about a category of animal or thing, one can use the to indicate the category:
The lion is a noble beast.
That use of "the" is more formal. However, the meaning of the example with the and the meaning using A lion or Lions would be the same thing. If you were writing a science article about a genus of plant, you would probably use:
The lily is native to [etc]. For example. Rather than Lilies or A lily.
The mobile telephone was a late, 20th c. invention.
Lastly, in speaking, one does not generally use the "the" version unless you are giving a speech.