ARFF (Attribute-Relation File Format) is a human-readable ASCII file format for machine learning datasets.

You can think of it as a more structured CSV.

Example

% Title: Blood Transfusion Service Center Data Set
%  
% Abstract: Data taken from the Blood Transfusion Service
% Center in Hsin-Chu City in Taiwan.
%
% This is a classification problem.

@relation blood-transfusion-service

@attribute V1 numeric
@attribute V2 numeric
@attribute V3 numeric
@attribute V4 numeric
@attribute Class {1,2}

@data
2,50,12500,98,2
0,13,3250,28,2
1,16,4000,35,2
2,20,5000,45,2
1,24,6000,77,1
4,4,1000,4,1