Masculine vs. Feminine

Everything in Spanish is either male or female. Yes, that includes tables, windows, and pencils. At first this doesn't seem to make a lot of sense, but you get used to it after a while.

Because everything has a specific gender, you have to use a specific article when talking about them.

the
el - masculine singular
la - feminine singular
los - masculine plural
las - feminine plural

a/an
un - masculine singular
una - feminine singular
unos - masculine plural
unas - feminine plural

Examples:
la sala de clases - the classroom
la pizarra - the chalkboard
el pupitre - the student's desk
la mesa - the table
el libro - the book
la hoja de papel - the sheet of paper
el bolígrafo - the pen
el lapíz - the pencil

So, whenever you learn a vocab word in Spanish, you also have to remember which gender it is...