Gemelo vs. mellizo

The Spanish words 'gemelo' and 'mellizo' both translate to 'twin' in English. However, they describe different types of twins.

Gemelo

This word is used to describe 'identical twins', also known as 'monozygotic twins'. These are twins that develop from a single fertilized egg, resulting in two individuals with identical genetic material.
María y Ana son gemelas.
(Maria and Ana are identical twins.)
Los gemelos tienen exactamente el mismo ADN.
(Identical twins have exactly the same DNA.)

Mellizo

This word refers to 'fraternal twins', or 'dizygotic twins'. These are twins that develop from two different eggs fertilized by two different sperm cells, resulting in two individuals with different genetic material.
Carlos y Pedro son mellizos, pero no se parecen mucho.
(Carlos and Pedro are fraternal twins, but they don't look very much alike.)
Los mellizos pueden ser de diferentes sexos.
(Fraternal twins can be of different sexes.)

Summary

'Gemelo' and 'mellizo' both describe pairs of siblings born at the same time, but they are used to differentiate between identical and fraternal twins. While 'gemelos' originate from the same egg and have identical DNA, 'mellizos' come from two separate eggs and have different DNA.