It's a question that confuses even medical professionals, largely because the definitions are constantly changing.
In a very broad sense, lymphomas and leukemias describe cancers that derive from blood-borne cells.
To that end, it used to be easier to understand the difference between leukemia, lymphoma and solid tumor cancers because, simply put, it was thought that blood cancers don't develop solid tumors. However, several subtypes of lymphoma are capable of developing solid tumors, nullifying this easy classification.
Also, there is the question of the type of blood-borne cell in which the cancer originated.
For starters, if a cancer derives from a lymphoid cell—-this includes lymphoblasts, lymphocytes, immunoblasts, plasma cells, follicle center cells-—then it should first be called a Lymphoid neoplasm. This is a relatively new term adopted by the latest edition of the American Joint Committee on Cancer (AJCC), the final word on cancer terminology and staging.
If it is indeed a Lymphoid Neoplasm, then a few other determinations are to be made based on how the cancer presents itself:
There are many reasons that are emerging in research and clinical trials across the world that suggest that there is no de facto difference between a lymphoma and leukemia—rather, they are merely two different phases of the very same developing disease.