A functor $F:C\to D$ is conservative if it is “isomorphism-reflecting”, i.e. if $g:a\to b$ is a morphism in $C$ such that $F(g)$ is an isomorphism in $D$, then $g$ is an isomorphism in $C$.
Sometimes conservative functors are assumed to be faithful as well. If $C$ has, and $F$ preserves, equalizers, then conservativity implies faithfulness.
See conservative morphism for a generalization to an arbitrary 2-category.
Let $\mathcal{C}$ be a category with pullbacks. Given any morphism $f \colon X \longrightarrow Y$ in $\mathcal{C}$ write
for the functor of pullback along $f$ between slice categories (base change). If strong epimorphisms in $\mathcal{C}$ are preserved by pullback, then the following are equivalent:
$f$ is a strong epimorphism;
$f^\ast$ is conservative.
(e.g. Johnstone, lemma 1.3.2)
Let $K : J \to C$ be a diagram in $X$ whose limit $\lim K$ exists and such that $\lim F\circ K \simeq F \lim K$. Then if $const_\theta \to K$ is a cone in $F$ that is sent to a limiting cone $F const_\theta$ in $D$, then by the universal property of the limit in $D$ the morphism $F( const_\theta \to \lim K)$ is an isomorphism in $D$, hence must have been an isomorphism in $C$, hence $const_\theta$ must have been a limiting cone in $C$.
The arguments for colimits is analogous.
Geun Bin Im, George Maxwell Kelly, Some remarks on conservative functors with left adjoints, J. Korean Math. Soc. 23 (1986), no. 1, 19–33, MR87i:18002b, pdf; On classes of morphisms closed under limits, J. Korean Math. Soc. 23 (1986), no. 1, 1–18, Adjoint-triangle theorems for conservative functors, Bull. Austral. Math. Soc. 36 (1987), no. 1, 133–136, MR88k:18005, doi
For an example of a conservative, but not faithful, functor $f: A\to Set$ having a left adjoint see Example 2.4 in