Fruit or fruits

Fruit and Fish are both singular and plural But when one wants to emphasize that there are different types, species, or kinds - the "s" is added for this emphasis. So while one might look at a varity of fruit or aquarium with a variety of fish. One can correctly say "that's a bunch of fruit" or "that tank is full of fish" but if one want to emphasis the different kinds - it is better to say "there are many fruits available here" and "this lake is teaming with a variety of fishes." but if you just say "there are so many types of fish in this lake" it is not wrong and no one will misunderstand.

“Fruit” can either mean a single unit like an apple or a banana, or the entire food group in general. The former is countable, and so the plural is “fruits.” The latter is uncountable, and so has no plural.

It works similarly to 'fish' and 'fishes'. 'fish refers to all fish in general; 'fishes' refers to a plural of individual fishes, or a plural of different types of fish. In both cases there situations where both forms can be consistently used. 'there are fish that use tools' and 'there are fishes that use tools'  are both syntactically correct. 'there is fruit with seeds on the outside' and 'there are fruits with seeds on the outside' are also both syntactically correct. 

Fruit or fruits is kind of like fish and fishes? If you're just talking about a pile of fruit it's fruit but if youre talking about different types of fruits it's fruits. Like "there is a bowl of fruit" and "peaches, cherries, and plums are fruits with stones".

Another less-common usage not mentioned is the meaning of "the benefits gained from", usually in "the fruits of your labor."

it's a bit like fish, whose plural is fish or fishes

It's like fish. If you've got multiple of the same kind of fish or you're just talking about fish in general, it's fish. If you're talking about several species of fish, it's fishes. Fruit is the same.

This is probably a British vs American debate. But, the word “fruit” exists in idioms like the “fruit of your labor”.

Many commenters say things like "you can use 'fruits' for multiple types of fruits". This is misleading, because we usually use "fruit" for this as well. (e.g. "I had fruit for breakfast." "I'm going to the store to buy some fruit." "This cereal is full of dried fruit.") A non-native will often say, "Well, I ate several types of fruit for breakfast, so I used the plural." More context is needed. The plural "fruits" is usually used in written, not spoken, English and has a bureaucratic flair. The signs in the grocery store often read "fresh fruits". "Fruits" is also (obviously?) used when you want to attach a number (e.g. "Our smoothie features nine fruits and vegetables."). This still has bureaucratic hints, i.e, you might see it on a sign or label, but you probably wouldn't say it. Sometimes you might use fruits conversationally (e,g "Mangoes and guavas are my favorite fruits."), where "fruits" can be *naturally* replaced with "kinds of fruit". "Fruits" is used in the pair "fruits and vegetables" for reasons of parallelism. I'm sure I missed a lot and have avoided uses for agriculture, biology or metaphors (e.g. the fruits of my labor).

As others have said, you can use both forms—“fruit” and “fruits” (as well as “fish” and “fishes”). But I think we should note that using the pluralized forms of each of these words is *slightly* archaic, poetic, or formal. In English, the pluralized forms evoke a slightly older time or a more literary sound. If you say “all the fish in the sea”, you’re just using an idiom, but if you say “all the fishes in the sea” you sound a little more elevated, old fashioned, or as if you’re quoting scripture. Similarly, if you say “enjoy the fruit of your labor” it’s more ordinary and idiomatic, but “enjoy the fruits of your labor” sounds slightly more like you’re quoting a famous phrase. Another note: there are certain constructions in English where only one version will do. For instance, the common phrase “fruits and vegetables” would sound quite odd to an American English speaker if the word “fruit” was used instead. The construction is common enough that the form has to be adhered to. But in British English, the phrase is (I believe) “fruit and veg”, is it not? And once again the common phrase construction should be followed.

The plural of fruit is only used when fruit is used to mean "kinds of fruit." If you substitute "kinds of fruit" and it doesn't make sense in context then it needs to be fruit not fruits. E.g., "The store sells many different kinds of fruit" = "The store sells many different fruits."

Fruits is plural, fruit is singular. A banana is a fruit. Bananas and apples are fruits. But you can use fruit as plural. Fruit is a noun it is both countable and uncountable. Fruits is typically used when you want to convey that there are multiple types of fruits but you could also use fruit in the same way.

Fruit can be a countable noun when talking about varieties of fruit: orange is a fruit; grapes, berries and lemons are fruits; this smoothie contains the juice of six fruits including mangoes, apples and pears. Fruit can not really be a countable noun when talking about individual objects. We don't say 'There is a fruit in the bowl', 'I ate three fruits'; we might say 'there is a piece of fruit in the bowl' or 'I ate three pieces of fruit'. VERY rarely it might be a countable noun for individual things, but probably only in technical or botanical kind of contexts: 'these apples are the fruits of this plant'. In some idiomatic phrases, 'fruits' might crop up as a plural: 'Enjoy the fruits of your labor.' Not entirely sure why that seems acceptable there; the phrase has biblical origin (Psalm 128:2) but none of the main translations give it as 'fruits of your labor', some say 'fruit of your labor'. But fruit is most commonly a **mass** **noun** and is really talking about the 'substance' of fruit. In fact that's how it's acting when we talk about 'a piece of fruit' - similar to 'sand' or 'ice' or 'water' or 'lego', these are mass nouns that describe the material something is made of. 'This tastes of fruit'; 'the bowl is full of fruit'; 'I ate loads of fruit'; same kind of structure as 'this bowl is full of sand', 'this bowl is full of ice' or 'this bowl is full of water' or 'this bowl is full of lego'. To count these you need a countable noun - there's thousands of grains of sand in the bowl (not thousands of sands); three chunks of ice in the bowl (not three ices); fifteen pieces of lego in the bowl (not fifteen legos no matter how much you want that to be the case); three pieces of fruit in the bowl (not three fruits).' This mass-noun form is really common with food types. Same logic applies to 'meat' and 'cheese': Lamb, beef and chicken are all meats; cheddar and gouda are cheeses. There's a selection of meats, cheeses, and fruits on the platter. But not "I'd like some meats please" or "I'd like some cheeses please" or "I'd like some fruits please". We eat meat, cheese and fruit, because they are a 'substance', not a countable thing.

I eat fruit - refers to fruit in general; I like banana and pineapple and mango Fruit is nutritious - again, non-specific: all of them are nutritious (even the ones, like melon, that i don't like)