Tropical is a rather loose term, but generally it refers to species which prefer a minimum temperature of 72°F or above. I make the note, "or above" as some species would be very unhappy even at this temperature and need water temperatures above 80°F. However most fish do well in temperatures in and around the mid 70°s and with a neutral pH.
Another term which is quite common is "Community Tank" or "Community Fish", and simply refers to compatible species which will live together in the same tank. Size, temperature, feeding habits, temperament, pH and water hardness are all important considerations if your pets are to co-exist and if deaths are to be avoided. Many people don't realise that for many fish their normal diet is other smaller species. A common question is "Will it eat my other fish?", and while there is no universal answer, it is a fair generalisation to say that any fish will eat any other fish which it can get into its mouth.
However, don't be put off by these issues as the vast majority of the fish available in the shop will co-exist quite peacefully with other of similar size. Those which are aggressive, or which need special conditions, are familiar to the staff and we will always mention these points when you ask to purchase such specimens.