Varieties cut by quadrics Is there a characterization of the class of varieties which can be described as an intersection of quadrics, apart from the taulogical one?
Lots of varieties arise in this way (my favorite examples are the Grassmanianns and Schubert varieties and some toric varieties) and I wonder how far can one go.
 A: Let $X\subset\mathbb{P}^N$ be a quadratic smooth variety of dimension $n$ and condimension $c$. Then:


*

*if $n\geq c$ then $X$ is Fano.

*If $n\geq c+1$ the $X$ is covered by lines. Let $x$ be a general point of $X$ and let $\mathcal{L}_x\subset\mathbb{P}^{n-1} = \mathbb{P}(T_xX)$ be the variety parametrizing lines in $X$ passing through $x$. Then $\mathcal{L}_x\subset\mathbb{P}^{n-1}$ is scheme theoretically defined by $c$ idependent quadratic equations.

*If $n\geq c+2$ then: $X\subset\mathbb{P}^N$ is a complete intersection $\Leftrightarrow$ $\mathcal{L}_x\subset\mathbb{P}^{n-1}$ is a complete intersection $\Leftrightarrow$ $dim(\mathcal{L}_x)=n-c-1$.

*(Hartshorne's conjecture) If $n\geq 2c+1$ then $X$ is a scheme theoretical complete intersection.

*If $X$ is prime Fano (i.e. $X$ is Fano and $Pic(X) = \mathbb{Z}\left\langle H\right\rangle$) and the Fano index satifies $i(X)\geq\frac{2n+5}{3}$, then $X$ is a complete intersection.


This has been a breakthrough in the direction of Hartshorne's conjecture on complete intersections. Before it was known just for very special varieties and for Fano varieties of codimension two. You can find all of this in this paper: http://arxiv.org/abs/1209.2047.
A: In fact the answer is in some sense tautological: every projective variety can be realized as a scheme-theoretic intersection of quadrics!  See e.g.
D. Mumford,   "Varieties defined by quadratic equations", Questions on algebraic varieties, C.I.M.E. Varenna, 1969 , Cremonese  (1970)  pp. 29–100,
for quantitative refinements of this question. 
A: Here is an  easy hands-on way to define  any  variety as being cut out by quadrics.
Write your variety as the zero locus of a bunch of  polynomials. 
Mark  each   monomial that occurs in any one of these polynomials 
and which   has   degree higher than two.  We are going to add new variables
and new quadratic equations to turn that monomial into a degree two monomial. 
For example if the monomial $x y^2$ occurs in one of your polynomials , add a new  variable $z$ and add the equation  $z = xy$.
Then  rewrite all occurrences of $xy^2$  in any  of your polynomials as $zy$. 
To take another example, if the monomial  $x^5$ occurs then add two new variables, say $u$ and $v$, together with the   two new  quadric equations   $u = x^2$
$v = u^2$ and the replace all occurrences of  $x^5$ by $xv$.
  Just keep on tacking new variables,
 and new equations until you've turned  all of your original monomials
into either quadratic or linear monomials. Now you've got your variety 
sitting in a much bigger space due to all the new variables,
but its defining  polynomials are now all quadratic plus linear.
(If you don't like the linear terms arising , play the homogenization game.)  
I learned this trick in some two page paper by a Russian  showing every compact manifold is
diffeomorphic to one defined by quadratic equations.  That paper in turn is related to 
a theorem attributed to Milnor (or Thurston) asserting that every manifold can be realized
by planar linkages. Sorry I don't have the references.    
A: As Pete already indicated, Mumford's theorem says that for any projective variety $X\subset \mathbb P^n$, its Veronese emberdding $v_d(X)\subset \mathbb P^N$ is cut out by quadrics, for $d\gg0$. So a reasonable question is for the variety with a fixed projective embedding (such as Grassmannians in the Plucker embedding and not in some other random embedding).
For this latter more meaningful question, many "combinatorial" rational varieties, such as Grassmannians, Schubert varieties (as you pointed out), flag varieties, determinantal varieties, etc., are cut out by quadrics.
For the "non-combinatorial", non-rational varieties, the most classical result is Petri's theorem: a smooth non-hyperelliptic curve of genus $g\ge 4$  in its canonical embedding is cut out by quadrics, with the exceptions of trigonal curves and plane quintics.
There is a vast generatization of this property: $X\subset \mathbb P^n$ satisfies property $N_p$ if the first syzygy of its homogeneous ideal $I_X$ is a direct sum of $\mathcal O(2)$, the second syzygy is a direct sum of $\mathcal O(3)$, etc., the $p$-th syzygy is a direct sum of $\mathcal O(p+1)$. In this language, $X$ is cut out by quadrics is equivalent to the property $N_1$.
A 1984 Green's conjecture is that a smooth nonhyperelliptic curve satisfies $N_p$ for $p=Cliff(X)-1$, where $Cliff(X)$ is the Clifford of $X$. This has been proved for generic curves of any genus by Voisin (in characteristic 0; it is false in positive characteristic).
Another notable case: the ideal of $2\times 2$ minors of a $p\times q$ matrix has property $N_{p+q-3}$. 
A: It seems impossible to give a better answer than Pete L. Clark's but those passing by might appreciate a sketchy proof.
If $X \subset \mathbb P^n$ is a projective variety then it  is the intersection of finitely many hypersurfaces of degree at most $k$. If we consider the natural morphism from $\mathbb P^n$ to $\mathbb P H^0(\mathbb P^n,\mathcal O_{\mathbb P^n}(k))^{\ast}$ then the image of $X$ will be the intersection of the image of  $\mathbb P^n$ with a finite number of hyperplanes.
Being the image of $\mathbb P^n$ itself a intersection of quadrics, it follows that any projective variety can be expressed as the intersection of quadrics and hyperplanes.
A more condensed version of the argument above can also be found here.
A: As Pete L. Clark says, if you can Veronese your line bundle then the answer is "all of them".
So a more interesting question may be: for which varieties M does every ample line bundle give an embedding defined by quadrics?
The best sufficient condition I know is that MxM have a Frobenius splitting w.r.t. which the diagonal is compatibly split. See Brion and Kumar's book about Frobenius splitting, and Sam Payne's article, which discusses the toric case. The first shows that it's true for flag manifolds, and the second that it's not true for all Schubert varieties.
