From Gerald Erichsen,
Your Guide to Spanish Language.
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Two of the most common conjunctions in Spanish — y (meaning "and") and o (meaning "or") — can change spelling and pronunciation based on the word that follows.

Both changes help keep the conjunction from blending into the following word. Y becomes e when it precedes a word that begins with the i sound, while o becomes u when it precedes a word that begins with the o sound. Typically, then, the y becomes e when it precedes most words that begin with i- or hi-, and o becomes u preceding words starting with o- or ho-. Y does not change before words, such as hierba, that begin with the y sound, regardless of spelling.

Following are examples of correct usage:

• Reciben tratamiento cruel e inhumano. (They are receiving cruel and inhumane treatment.)
• Vendemos productos de limpieza e higiene. (We sell cleaning and hygiene products.)
• Está enteramente construido de nieve y hielo. (It is built completely of snow and ice.)
• Usamos punto y coma para separar las frases u oraciones que constituyen una enumeración. (We use a semicolon to separate phrases or sentences that make up a list.)
• No recuerdo si fue ayer u hoy. (I don't remember if it was yesterday or today.)