Classification problem

Predict a discrete valued output (yes/no) or (A, B, C, D)

What kind of an animal is it? Is it a cat, a dog, a tiger?

X: * height * weight * size of ears, * size of tail * color, color of the eyes * ... * OR the picture of an animal

Medical diagnosis

X: * Lab results * Images (CT, MRI, Ultrasound) * Patient metadata

Brest Cancer: Tumor size (x). Is it malignant or benign (y)? Two distinct possibilities. Given a tumor (and its size) what is the probability that it is malignant?
The Tumor size is a "feature". In other problems we might have many more features.
e.g. We might know the tumor size, the age of the patient, the gender of the patient, etc.

Iris species

Classical example from the paper of Ronald Fisher in 1936: "The use of multiple measurements in taxonomic problems"
y:
- Iris-setosa
- Iris-versicolor
- Iris-virginica
X:
- sepal and petal length and width (4 numbers)