Identical copy of a
chromosome, attached to the original at the
centromere.
The copy is produced during interphase right before mitosis and meiosis, while the genetic information is still in the form of chromatin.
During the anaphase of mitosis and meiosis the sister chromatids are pulled apart by the kinetochore microtubules attached to their centromere.
There are then 2 complete and separate copies of the DNA and the cell can finish dividing.
Having the two copies together helps to ensure the cell does not allocate an incomplete set of chromosomes to the daughter cells.
During meiosis, homologous sets of sister chromatids get together to form the tetrad, where they cross over. Again, having the copies of the chromosomes attached helps to ensure the correct DNA is passed to the correct spot.