The difference between bitcoins and every altcoin is their purposes or concepts and ideologies. The term altcoin which stands for alternative coin was made because this forum was made for the sole purpose of discussing about bitcoins only. But then the altcoins came which is why they became altcoins. but basically if you think about it, bitcoin is also an altcoin for every altcoin. And since this forum is the most successful one in this industry, people has adopted the term altcoins. hope this makes sense.