First, pick a commutative ring $k$ as the "ground field". Everything I say will be $k$-linear, e.g. "algebra" means "unital associative algebra over $k$". Then recall the following construction due to Grothendieck:


Let $A$ be a commutative algebra, and $X$ an $A$-module. Then the **differential operators on $X$** is the filtered algebra $D = D(A,X)$ given inductively by:
$$ D\_{\leq 0} = \text{image of $A$ in }\hom\_k(X,X) $$
$$ D\_{\leq n} = \{ \phi \in \hom\_k(X,X) \text{ s.t. } [\phi,a] \in D\_{\leq n-1}\\, \forall a\in A\} $$
$$ D = \bigcup\_{n=0}^\infty D\_{\leq n} $$
(**Edit:** In the comments, Michael suggests that $D\_{\leq 0} = \hom\_A(X,X)$ is the more standard definition, and the rest is the same.)


Then the following facts are more or less standard:


* If $A$ acts freely on $X$, then $D\_{\leq 1}$ acts on $A$ by derivations. (It's always true that $D\_{\leq 1}$ acts on $D\_{\leq 0}$ by derivations; the question is whether $D\_{\leq 0} = A$ or a quotient.) If $X = A$ by multiplication, then $D\_{\leq 1}$ splits as a direct sum $D\_{\leq 1} = \text{Der}(A) \oplus A$.
* If $k=\mathbb R$, $M$ is a finite-dimensional smooth manifold, and $A = C^\infty(M) = X$, then $D$ is the usual algebra of differential operators generated by $A$ and $\text{Vect}(M) = \Gamma(TM \to M)$.
* If $A,X$ are actually sheaves, so is $D$.


Thus, at least in the situation where $A = X = C^{\infty}(M)$, the algebra $D$ is acting very much like the universal enveloping algebra of $U (\text{Vect}(M))$; in particular, the map $U(\text{Vect}(M)) \to D$ is filtered and is (almost) a surjection: it misses only the non-constant elements of $A$. So when $A = X = C^\infty(-)$ are sheaves on $M$, it's very tempting to think of $D$ as a sheafy version of $U(\text{Vect}(-))$. Note that $U(\text{Vect}(-))$ is not a sheaf: its degree $\leq 0$ part consists of constant functions, not locally constant, for example, and there are non-zero elements in $U\_{\leq 2}$ that restrict to $0$ on an open cover. I think that it cannot be true that the sheafification of $U(\text{Vect}(-))$ is $D$, as the sheafification of $U\_{\leq 0}$ is the locally-constant sheaf, not $C^\infty$.


So: is there a description of $D$ that makes it more obviously like a universal enveloping algebra? E.g. is there some adjunction or other categorical description? Is it really true that $D$ is a "sheafy" version of $U$ in a precise sense, or is this just a chimera?



Is there a "categorical" description of Grothendieck's algebra of differential operators?
The construction you're looking for is the universal enveloping algebra of a [Lie](http://en.wikipedia.org/wiki/Lie_algebroid) [algebroid](http://ncatlab.org/nlab/show/Lie+algebroid).


A Lie algebra is a Lie algebroid on a point. It's universal enveloping algebra as an algebroid is the same as the usual.


Every smooth manifold has a Lie algebroid structure on its tangent bundle, which differential operators are the universal enveloping algebra of.


Another cool examples is that TDOs (sheaves of twisted differential operators) can be constructed from central extensions of the tangent Lie algebroid by the structure sheaf (you take UEA, and then identify the two copies of the structure sheaf).


(I'll just note: Mariano's answer and mine are essentially equivalent. See the nLab page linked above for brief explanation).


