Underfitting:

• The model cannot capture the complexity of the data
• Model too simple to describe the data

Overfitting:

• Fitting too specific to the dataset, loosing generality
• Model too complex to describe the data

Example: Prepare for an exam by memorizing the answers to all the questions appeared in previous exams

• If same questions appear ➜ score very well
• If different questions ➜ score very bad