Why are some Spanish words masculine and others feminine?

Many languages, for example Spanish, French, German and Russian give their nouns (naming words e.g. dog, table, grandmother) a gender. English is the exception here, so lucky us, but this does make it a bit more difficult when learning a language. 

Masculine nouns (e.g. el perro, el padre) use 'el' for 'the' and 'un' for 'a/one', wheras feminine nouns (e.g. la tortuga, la madre) use 'la' and 'una'. 

When talking about people, this is easy: if you have a brother, he's 'el hermano' and a sister, 'la hermana'. The same goes for auntie and uncle: la tía, el tío.

But for most nouns, you just have to learn it and often the word will give you a clue as to its gender. 

Think of 'o' in Spanish as masculine and 'a' as feminine. If a word ends in one of those two letters, the rule almost always applies. So 'el banco' (the bank) is masculine but 'la mesa' (the table) is feminine. This might seem strange but stick with it! 

Other suffixes (word endings) also give hints to the gender, for example words ending in 'ción' or 'dad', e.g. la información, la ciudad are always feminine. 

Don't worry too much about this, it's a strange concept but as your understanding grows, you'll have a natural feeling for the gender of a word - i promise!