Leukemia is tumor of blood cells and blood cells precursors not only lymphocytes it can be anything for example in CML you have proliferation of all cell lineages.
Dx, Bone Marrow biopsy
Lymphoma is a tumor of lymphoid tissue mainly sub-classified into Hodgkin's and non-Hodgkin's
Dx, Lymph node biopsy
Remember it's these lymphoid tissues were lymphocytes and precursors get matured.
Perhaps u r confused because of the fact that sometimes blood tumors may have elements of both for example T-Cell ALL closely resembles Lymphocytic lymphoma with thymic shadows.