The BLAS (Basic Linear Algebra Subprograms) are high quality "building block" routines for performing basic vector and matrix operations. Level 1 BLAS do vector-vector operations, Level 2 BLAS do matrix-vector operations, and Level 3 BLAS do matrix-matrix operations. Because the BLAS are efficient, portable, and widely available, they're commonly used in the development of high quality linear algebra software, LAPACK for example.
Usually, the vendor of a computer system makes available highly optimized BLAS subroutines. Otherwise, an automatic optimized BLAS can be build, using the ATLAS package.