A tonsil consists of an epithelially-lined crypt (invaginated pocket) surrounded by dense clusters of lymph nodules, each with a germinal center where lymphocytes proliferate. The nodules are embedded in a mass of diffuse lymphoid tissue that consists of lymphocytes migrating to and from the germinal centers.
This tonsil of the soft palate is lined by stratified squamous epithelium, which is so heavily infiltrated with lymphocytes that the epithelium itself is obscured.
The lymphoid tissue of the tonsils is similar to that of Peyer's patches and appendix. These structures, together with other more diffuse lymphoid tissue, constitute the Gut-Associated Lymphoid Tissues, or GALT.
For more on GALT (or, more generally, MALT for Mucosa-Associated Lymphoid Tissues), consult your histology text (e.g. pp. 134-5 in Stevens & Lowe).