Vectors come in two flavors: column vectors and row vectors. For example, matrix a is a column vector, and matrix a' is a row vector.
a =
11
12
33
a' =
11 12 33
We use lower-case, boldface letters to represent column vectors. And since the transpose of a column vector is a row vector, we use lower-case, boldface letters plus a prime to represent row vectors. Thus, vector b would be a column vector, and vector b' would be a row vector.