“Seseo” consists of pronouncing the letters “c” (before “e” and “i”) and “z” with the sound normally associated with the letter “s”. If someone speaks with seseo, they would say “serésa” for “cereza” (cherry), “sierto” for “cierto” (true), and “sapáto” for “zapato” (shoe).
Seseo is used generally throughout Latin America and in the Caray Islands and part of Andalucía, as well as certain areas of Murcia and Badajoz. You can also find examples of seseo in the working classes of Valencia, Cataluña, Mallorca, and the Basque Country, when they speak Spanish, as well as some rural areas of Galicia. Seseo in Southern Spain (Andalucía and The Canary Islands) and Latin America are completely accepted as standard use.
“Ceceo” occurs with the letter “s” is pronounced with a sound similar to the sound of the letter “z” in speech in Central, Northern, and Eastern Spain. Someone who uses ceceo would say “káza” for “casa” (house), “zermón” for “sermón” (sermon), “perzóna” for “persona” (person). Ceceo is a dialect phenomenon of certain areas of Southern Spain and is not as widely used as seseo.
Source: Real Academia Española