Rank 2 vector bundles over $\mathbb CP^2$ Is there any classification of the rank 2 complex vector bundles over $\mathbb CP^2$ up to diffeomorphism?   
 A: As additional comment, We have Cartan-Serre theorem for construction of rank 2 vector bundles on projetive varieties.
Let $X$ be a complex manifold, $L_1, L_2 ∈ Pic(X)$ are line bundles,
$Z ⊂ X$ with $codim_X(Z) = 2$. Under some cohomological conditions, the
sheaf $E$ sitting in 
$$0 \to L_1 \to E \to L_2 \otimes J_Z → o$$
is locally free. Moreover, If $X$ is projective, any rank-2 holomorphic vector bundle can be constructed this way
A: The question was essentially answered by Sasha's comment, but I will give a couple of additional/alternative references/remarks.
As a first remark, the continuous and smooth classifications coincide, this is discussed e.g. in the answers to this MO-question: From Topological to Smooth and Holomorphic Vector Bundles
The continuous classification of rank 2 complex vector bundles on $\mathbb{CP}^2$ is given exactly in terms of Chern classes. This can essentially be proved by obstruction theory (compute homotopy groups of $BU(2)$, identify the lifting classes in $H^{2i}(X;\mathbb{Z})$ with the Chern classes). 
More information can be found in 


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*F.P. Peterson. Some remarks on Chern classes. Ann. Math. 69 (1959), 414-420. 


Theorem 3.1 of that paper shows that there is an abelian group structure on the set of isomorphism classes of rank 2 bundles on $\mathbb{CP}^2$. Theorem 3.2 shows that a rank $2$ vector bundle on $\mathbb{CP}^2$ is trivial if and only if its Chern classes are trivial. (The triviality here is in the topological or the smooth category.)
The realizability of Chern classes $c_1,c_2$ by rank 2 vector bundles can also be derived from the obstruction theory. There is also a classical reference constructing rank 2 vector bundles on algebraic surfaces with prescribed Chern classes:


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*R.L.E. Schwarzenberger. Vector bundles on algebraic surfaces. Proc. London Math. Soc 11 (1961), 601-622.


This gives a more explicit construction which alternatively is known under the name Hartshorne-Serre correspondence.
Finally, since the question is tagged algebraic geometry, let me point out that the algebraic or holomorphic classification of rank 2 bundles on $\mathbb{CP}^2$ is very complicated. After fixing Chern classes and an additional invariant called the splitting type, the resulting moduli space of bundles has infinitely many irreducible components (so that there are uncountably many algebraic structures on the trivial rank 2 bundle). 
