This might be a basic question:
What is the difference between bias and underfitting? And, analogously, what is the difference between variance and overfitting? Do the terms of each pair mean the same thing? If not, what is the difference?
Solved – Is bias the same as underfitting and variance the same as overfitting
biasmodel-evaluationoverfittingvariance
Related Solutions
All are metrics to find the best model: You would like to have an unbiased minimum variance estimator related to the validation interval - and thus neither over nor underfitted. But how to balance these metrics is up to your application / context.
Bias: The error you have for certain even if you can use an infinite number of cases / records. Usually you try to get unbiased models.
Variance / Significance: Relates to the probability that the true relationships between your variables are trivial (e.g. zero) and your model sees an accidental and purely randomly generated data pattern. Variance and bias are usually independent.
Overfitting: Is related to the variance, but it's not the same. If you have a large data matrix you may fit a model with a large number of covariats and many parameters may have a small variance. Nevertheless if you split off 20% of the training material and make a prediction with your model on that validation data, the model may predict worse. That's overfitting: Your model fitted relationships, which aren't randomly within your full data set, but aren't systematic and stable for extrapolations outside the training data set.
Underfitting goes usually with a biased distribution of your residuals: Your model is insufficiently specified. (Relationship between target and covariat is quadratic, but you fit linear). Underfitting can result in a bias, but doesn't have to do. In any case it results in a non-minimal variance. Hope that helps a little bit.
You would find the answer in the ISLR book Chapter 2.2.1
in short, plz check out pictures 
and
(from the book).
Too flexible model (overfitting) loses generalisation ability by remembering too much noise - increasing variance (second picture - greenish waving non-linear approximation for linear dataset), while inflexible model (underfitting) cannot describe dataset properly (upper picture - beige color line tries to approximate non-linear dataset)
Best Answer
They do not exactly mean the same thing, but they are correlated in the following manner:
Over fitting occurs when the model captures the noise and the outliers in the data along with the underlying pattern. These models usually have high variance and low bias. These models are usually complex like Decision Trees, SVM or Neural Networks which are prone to over fitting.
Under fitting occurs when the model is unable to capture the underlying pattern of the data. These models usually have a low variance and a high bias. These models are usually simple which are unable to capture the complex patterns in the data like Linear and Logistic Regressions.