The Spanish language has a gender system – nouns (and the adjectives and articles that refer to them) are either masculine or feminine. So (in my examples, words in Spanish are bold; words in English are bold italic):
un = a (masculine)
una = a (feminine)
un libro = a book
una casa = a house
There are lots of rules and lots of exceptions about which nouns are masculine and which are feminine, but a basic start is that nouns ending in -o tend to be masculine, and nouns ending in -a tend to be feminine. The contrast is obvious in, for example:
un gato = a (male) cat
una gata = a (female) cat
un hermano = a brother
una hermana = a sister
So what’s ‘uno’? Unfortunately, ‘uno’ is not the masculine equivalent of ‘una’, but is simply the number ‘one’, used without an attached noun:
¿Cuántas hermanas tienes? = How many sisters do you have?
Tengo una hermana = I have one sister
Tengo uno = I have one
Spanish is a Romance language, that is, a language derived from “Vulgar Latin”, the Latin spoken by ordinary people in the Roman Empire. Spanish is the most widely-spoken of the Romance languages; other well-known Romance languages (over a million speakers) are Portuguese, French, Italian, Romanian, Galician (spoken in the north-west of Spain), and Catalan (spoken in the east of Spain, and the official language of Andorra). There are many less widely-spoken Romance languages, including some spoken in Spain, such as Asturian, Leonese, and Aragonese. Spanish is also called Castilian / castellano, sometimes to distinguish it from other languages spoken in Spain.