Overfitting in machine learning models occurs when a model performs exceptionally well on training data but poorly on unseen data, often due to the model's complexity. Techniques such as cross-validation help detect overfitting, while strategies like simplifying models, feature selection, early stopping, and regularization combat it. Regularization, particularly L1 (Lasso) and L2 (Ridge), introduces bias to prevent overfitting by modifying the model's objective function. L1 regularization tends to create a sparse model by assigning zero weights to less significant features, inherently performing feature selection, whereas L2 regularization keeps all weights small but non-zero, which is computationally less expensive. The choice between L1 and L2 depends on the specific problem requirements, such as the need for model interpretability, robustness to outliers, and computational constraints.