"D" means Dairy, so must not be eaten with a meat meal.
"DE" means Dairy Equipment (no actual dairy in ingredients, hence it can be eaten even after a meat meal).
"Parve" or "Pareve" means the product is "neutral" (i.e. non-dairy and non-meat), so can be eaten with either type of meal.
"P" means Passover, i.e. it is Kosher for all year including Passover. (Note: "P" never designates Pareve.) On Passover, use ONLY hechshers displaying "P", "Kosher for Passover", "Kosher l'Pesach" or similar.
"Cholov Yisroel" means that Kosher supervised milk is used in ingredients.
"Pas Yisroel" indicates Jewish baked foods.
"K": A plain K (with or without serif) that has no other component to its design is a symbol that is not accepted by VHD without specific prior approval. Since a letter of the alphabet cannot be trademarked, anyone can print it on packaging with or without kosher oversight.