Grothendieck conjectured that every Weil cohomology theory factors uniquely through some category, which he called the category of motives. For smooth projective varieties (over some field $k$) such a category was given by Grothendieck himself, called the category of pure Chow motives. For general smooth varieties the category is still conjectural, see at mixed motives.
Fix some adequate equivalence relation $\sim$ (e.g. rational equivalence). Let $Z^i(X)$ denote the group of $i$-codimensional algebraic cycles and let $A^i_\sim(X)$ denote the quotient $Z^i(X)/\sim$.
Let $Corr_\sim(k)$, the category of correspondences, be the category whose objects are smooth projective varieties and whose hom-sets are the direct sum
where $(X_i)$ are the irreducible components of $X$ and $n_i$ are their respective dimensions. The composition of two morphisms $\alpha \in Corr(X,Y)$ and $\beta \in Corr(Y,Z)$ is given by
where $p_{XY}$ denotes the projection $X \times Y \times Z \to X \times Y$ and so on, and $.$ denotes the intersection product in $X \times Y \times Z$.
There is a canonical contravariant functor
from the category of smooth projective varieties over $k$ given by mapping $X \mapsto X$ and a morphism $f : X \to Y$ to its graph, the image of its graph morphism $\Gamma_f : X \to X \times Y$.
The category of correspondences is symmetric monoidal with $h(X) \otimes h(Y) \coloneqq h(X \times Y)$.
We also define a category $Corr_\sim(k, A)$ of correspondences with coefficients in some commutative ring $A$, by tensoring the morphisms with $A$; this is an $A$-linear category additive symmetric monoidal category.
The Karoubi envelope (pseudo-abelianisation) of $Corr_\sim(k, A)$ is called the category of effective pure motives (with coefficients in $A$ and with respect to the equivalence relation $\sim$), denoted $Mot^eff_\sim(k, A)$.
Explicitly its objects are pairs $(h(X), p)$ with $X$ a smooth projective variety and $p \in Corr(h(X), h(X))$ an idempotent, and morphisms from $(h(X), p)$ to $(h(Y), q)$ are morphisms $h(X) \to h(Y)$ in $Corr_\sim$ of the form $q \circ \alpha \circ p$ with $\alpha \in Corr_{\sim}(h(X), h(Y))$.
This is still a symmetric monoidal category with $(h(X), p) \otimes (h(Y), q) = (h(X \times Y), p \times q)$. Further it is Karoubian, $A$-linear and additive.
The image of $X \in SmProj(k)$ under the above functor
is the the motive of $X$.
There exists a motive $\mathbf{L}$, called the Lefschetz motive, such that the motive of the projective line decomposes as
To get a rigid category we formally invert the Lefschetz motive and get a category
the category of pure motives (with coefficients in $A$ and with respect to $\sim$).
This is a rigid, Karoubian, symmetric monoidal category. Its objects are triples $(h(X), p, n)$ with $n \in \mathbf{Z}$.
When the relation $\sim$ is rational equivalence then $A^*_\sim$ are the Chow groups, and $Mot_\sim(k) = Mot_{rat}(k)$ is called the category of pure Chow motives.
When the relation $\sim$ is numerical equivalence, then one obtains numerical motives.
Yuri Manin, Correspondences, motifs and monoidal transformations , Math. USSR Sb. 6 439, 1968(pdf, web)
Tony Scholl?, Classical motives, in Motives, Seattle 1991. Proc Symp. Pure Math 55 (1994), part 1, 163-187 (pdf)
James Milne, Motives – Grothendieck’s Dream (pdf)
Minhyong Kim, Classical Motives: Motivic $L$-functions (pdf)
Bruno Kahn, pdf slides on pure motives
R. Sujatha, Motives from a categorical point of view, Lecture notes (2008) (pdf)
Section 8.2 of