What happens with code like this

#pragma omp parallel for
for (i=0; i<n; i++) sum += a[i]*a[i];

All threads can access the sum variable, but the addition is not atomic! It is important to avoid race between threads. So-called reductions in OpenMP are thus important for performance and for obtaining correct results. OpenMP lets us indicate that a variable is used for a reduction with a particular operator. The above code becomes

sum = 0.0;
#pragma omp parallel for reduction(+:sum)
for (i=0; i<n; i++) sum += a[i]*a[i];