Given any injective immersion f : N → M the image of N in M can be uniquely given the structure of an immersed submanifold so that f : N → f(N) is a diffeomorphism. It follows that immersed submanifolds are precisely the images of injective immersions.
An embedded submanifold (also called a regular submanifold) is an immersed submanifold for which the inclusion map is a topological embedding. That is, the submanifold topology on S is the same as the subspace topology.
Given any embedding f : N → M of a manifold N in M the image f(N) naturally has the structure of an embedded submanifold. That is, embedded submanifolds are precisely the images of embeddings.
There is an intrinsic definition of an embedded submanifold which is often useful. Let M be an n-dimensional manifold, and let k be an integer such that 0 ≤ k ≤ n. A k-dimensional embedded submanifold of M is a subspace S ⊂ M such that for every point p ∈ S there exists a chart (U ⊂ M, φ : U → Rn) containing p such that φ(S ∩ U) is the intersection of a k-dimensional plane with φ(U). The pairs (S ∩ U, φ|S ∩ U) form an atlas for the differential structure on S.
There are some other variations of submanifolds used in the literature. Sharpe (1997) defines a type of submanifold which lies somewhere between an embedded submanifold and an immersed submanifold.
Given any immersed submanifold S of M, the tangent space to a point p in S can naturally be thought of as a linear subspace of the tangent space to p in M. This follows from the fact that the inclusion map is an immersion and provides an injection
Suppose S is an immersed submanifold of M. If the inclusion map i : S → M is closed then S is actually an embedded submanifold of M. Conversely, if S is an embedded submanifold which is also a closed subset then the inclusion map is closed. The inclusion map i : S → M is closed if and only if it is a proper map (i.e. inverse images of compact sets are compact). If i is closed then S is called a closed embedded submanifold of M. Closed embedded submanifolds form the nicest class of submanifolds.
Manifolds are often defined as embedded submanifolds of Euclidean space Rn, so this forms a very important special case. By the Whitney embedding theorem any second-countable smooth n-manifold can be smoothly embedded in R2n.