Is there a subtle difference between these two concepts or are they basically the same thing? When I mean differentiated, I don't mean terminally differentiated.
When you look at the development of the embryo, at the beginning all cells are totipotent, meaning they can develop into any cell type of the body. This changes relatively fast by differentiation, which means that the totipotent cells develop into more specialized cell types, which then can not give rise to all cell types. So the different germ layers (meso, ecto- and endoderm) develop and from these all the cell types in our body. See the first figure for clarification (from here):
Lineage restriction means that you have a differentiated cell line, which can only further develop into cells of this line. So hematipoietic stem cells can only further develop into cells of the hematopietic line (lymphocytes, erythrocytes, basophiles and so on), like shown in the figure below (taken from here):