Lymphoid tissue is rich in lymphocytes (and accessory cells such as macrophages and reticular cells). The lymphoid tissue includes the lymph nodes, spleen, tonsils and adenoids and the thymus (an organ in the neck that is particularly large during infancy).
A tissue in medicine is not like a piece of tissue paper. It is a broad term that is applied to any group of cells that perform specific functions. A tissue in medicine need not form a layer. Thus, the bone marrow is also a tissue.