For all intents and purposes, English hasn't had them since the medieval ages (when English sounded, well, not a lot like English at all, and grammar was something more of a free-for-all than a coherent set of rules). German has three (marked by the infamous der, die and das articles), French has just two, while Polish has the full three, masculine, feminine and neuter. You guessed it, I'm talking about the gender of nouns, and in Polish it's a very important thing to learn.
The gender of a noun will not only determine the ending of an adjective associated with it, but also how its ending changes as you move from case to case to express different meanings. In the absence of articles like in German (a welcome trait of the Polish language), it is the various noun endings that usually indicate its gender, but beware, because there is a multitude of those obligatory grammatical exceptions to get your head round. It's also really important to remember that the character of a noun - that is, how you or me perceive it - has nothing to do with its gender (kot [cat] is masculine for example, one I always found odd perhaps because I'd only ever owned male dogs, but still, you get my drift).
Determining Noun Gender
Exceptions aside, there are a number of ways you can deduce the gender of a Polish noun 'at a glance', so to speak. This is because feminine nouns, masculine nouns, and neuter nouns all follow certain ending trends. It's worth getting to know these at least, because even if you can't get your head around all the exceptions, being able to conjugate according to these rough rules will give you a pretty good success rate with cases later on.
A noun stem is the part of a noun that we use to conjugate (so it’s useful to know how to find these). Simply put, it's the simplest form of a noun without any associated endings.
Masculine Noun Endings
Generally speaking, masculine nouns end in a consonant (remember kot?). This rule includes all consonants except the letter -ć, which is common to female nouns. In addition any noun depicting a particular animate object (notably mężczyzna 'man', but also professions like dentysta 'dentist'), or an abstract animate thing (idiota 'an idiot') that is not necessarily, but is potentially male, is also masculine regardless of ending.
Often, because masculine nouns frequently end in a consonant, their stems are the same as the full noun.
Feminine Noun Endings
Where consonants were the trend of masculine nouns, here vowels are the name of the game, especially the ending -a (kobieta 'woman', ulica 'street'). As we saw above, the soft sounding consonant ending -ć (kość 'bone') usually indicates a feminine noun, along with some other soft consonant endings (but these are by no means across the board).
Noun stems for feminine nouns are usually obtained by removing the ending vowel, but in the case of soft consonant endings, the stem may be identical.
Neuter Noun Endings
The general rule for neuter nouns is the vowel ending -e or -o, where the -e ending almost always follows a soft consonant. (Morze 'sea', okno 'window'.) But many polish nouns are increasingly being borrowed from their Latinate roots, and these retain their -um sounding endings (muzeum - 'museum').
As with feminine nouns the stems of neuter nouns are usually obtained by simply removing their ending vowel, while the endings to Latinate root neuter nouns can be unpredictable and tricky, so it's best to learn these individually.
Getting to know your noun ending trends should be one of the first steps you take in approaching Polish cases, which will require you to apply separate conjugations for each gender and their associated adjectives.