See the link on indexing:
In particular, if you supply only one subscript as in your first example, then MATLAB will use what is known as "Linear Indexing" in the link, and give you a column vector as a result with the elements being composed of the linear indexed elements you specified with somevector.
In your second example, you supplied two indexes. The result will be a sub-matrix composed of all the rows (via the : supplied for the first index), and the columns supplied by your somevector.
