Kraken is a k-mer based taxonomic classification tool that can classify input sequences using k-mers (short sequences usually of length 31) from input datafiles (e.g. fasta or fastq). Taken from Kraken website

Kraken is a system for assigning taxonomic labels to short DNA sequences, usually obtained through metagenomic studies. Previous attempts by other bioinformatics software to accomplish this task have often used sequence alignment or machine learning techniques that were quite slow, leading to the development of less sensitive but much faster abundance estimation programs. Kraken aims to achieve high sensitivity and high speed by utilizing exact alignments of k-mers and a novel classification algorithm.

Kraken requires a database of short sequences for classification, please refer to the Kraken webpage for further details, also refer to the paper: https://genomebiology.biomedcentral.com/articles/10.1186/gb-2014-15-3-r46

KronaTools(https://bmcbioinformatics.biomedcentral.com/articles/10.1186/1471-2105-12-385) can be used to visualised the taxonomy classification generated using Kraken.