The words "el" and "la" do not mean "this": they mean "the". The difference is grammatical gender. Every noun in Spanish has 2 properties: grammatical gender, and grammatical number. There are two grammatical genders: "masculine" and "feminine". There are two grammatical numbers: "singular" and "plural". All articles and many adjectives also have different forms for both grammatical gender and grammatical number. The gender and number of every article and every adjective must be the same as the grammatical gender of the noun that they modify. Masculine singular nouns require using the definite article "el" and the indefinite article "un". For example: el/un hombre (the/a man), el/un lápiz (the/a pencil) Masculine plural nouns require using the definite article "los" and the indefinite article "unos". For example: los/unos hombres (the/some men), los/unos lápices (the/some pencils). Most feminine singular nouns require using the definite article "la" and the indefinite article "una". For example: la/una mujer (the/a woman), la/una manzana (the/an apple), la/una hermana (the/a sister). However, feminine singular nouns that are accented on the first syllable and that start with the letter "a" or the letters "ha" require using the definite article "el" and the indefinite article "un". They are still feminine nouns and they still require using feminine adjectives. For example: el/un agua fría (the/a cold water), el/un hacha aguda (the/a sharp axe), el/un alma (the/a soul), el/un águila (the/an eagle). (This is similar to the rule in English for choosing "a" or "an".) Feminine plural nouns require using the definite article "las" and the indefinite article "unas". For example: las/unas mujeres (the/some women), las/unas manzanas (the/some apples), las/unas hermanas (the/some sisters), las/unas aguas frías (the/some cold waters), las/unas hachas agudas (the/some sharp axes), las/unas almas (the/some souls), las/unas águilas (the/some eagles).

@William The word "agua" is actually a feminine noun. However, because it begins with a stressed "a-" the article "el" is used. That's why it's "el agua" and "las aguas".

La and El mean the. You have to memorize the article. La is feminine El is masculine. Most of the time, an a at the end of the word means the word is feminine la manzana (the apple) or la casa (the house) but there are exception, El agua (the water), El problema (the promblem).