El is the Spanish masculine singular definite article.
Let's break that down. 'Definite article' is just a fancy was of referring to the word 'the'. In most Romance languages there are both feminine and masculine versions of the word 'the'. In Spanish, these are 'el' (masculine) and 'la' (feminine). Nouns also have a gender, and you must match the gender of the article with the gender of the noun; thus, "el perro" (the dog, masc.), but "la vaca" (the cow, fem.).
And likewise, there are both singular and plural versions of the definite article. The plural of 'el' is 'los' ("los perros" means "the dogs"). The plural of 'la' is 'las', ("las vacas").
'El' has entered peripherally into the English language when used in phrases like El Dorado, el cheapo, and El Nino. It also makes appearances in El Cid and El Salvador.
While 'la' appears in a handful of European languages, as far as I can find 'el' appears only in Spanish and Catalan.
The definite articles of most Romance languages, including those used in Spanish, come to us from Latin. While Latin did not use definite or indefinite articles, languages descending from it took the Latin ille, meaning 'that', to form their own definite articles. Both 'el' and 'la' are forms of ille, as are the definite articles in French, Italian, and Portuguese.
It is interesting to note that while English has roots in the Romance languages (i.e. French) and in Old English, which also had gendered nouns and articles, we don't use them at all. Old English se (masc.), seo (fem.), and þæt (nuet.) compacted into the single þe, and then into 'the'.
It should also be noted that the Arabic definite article, al-, is sometimes transliterated as el-. It is always used as a prefix, and not as a stand alone word.