Both of them mean "female". However κορίτσι (girl) we use for infants up to teenagers, while κοπέλα we use for older ages, from teenagers and up.
They also mean "girlfriend". Still we prefer to use "το κορίτσι μου" for younger ages (15-30 years) while "η κοπέλα μου" would be more appropriate for older ages (more than 20 years old).
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.