Describe the region $R$ consisting of the points $(a,b)$ of the cartesian plane for which both (possibly complex) roots of the polynomial $z^2+az+b$ have absolute value smaller than $1$.
To describe the region \( R \) consisting of the points \((a,b)\) in the Cartesian plane for which both (possibly complex) roots of the polynomial \( z^2 + az + b \) have absolute value smaller than \( 1 \), we need to analyze the conditions for the roots of the polynomial.

1. **Roots of the Polynomial**:
   The roots of the polynomial \( z^2 + az + b = 0 \) are given by:
   \[
   z_{1,2} = \frac{-a \pm \sqrt{a^2 - 4b}}{2}
   \]
   We need to ensure that both roots \( z_1 \) and \( z_2 \) have absolute values less than 1.

2. **Real Roots**:
   For real roots, the discriminant \(\Delta = a^2 - 4b \geq 0\).

   - **Condition for \(|z_1|, |z_2| < 1\)**:
     - The polynomial evaluated at \( z = -1 \) should be positive:
       \[
       f(-1) = 1 - a + b > 0
       \]
     - The polynomial evaluated at \( z = 1 \) should be positive:
       \[
       f(1) = 1 + a + b > 0
       \]
     - The vertex of the parabola \( z_m = -\frac{a}{2} \) should lie within the interval \((-1, 1)\):
       \[
       -1 < -\frac{a}{2} < 1 \implies |a| < 2
       \]

     Combining these conditions, we get:
     \[
     \begin{cases}
     a \in (-2, 2) \\
     b \leq \frac{a^2}{4} \\
     b > a - 1 \\
     b > -a - 1
     \end{cases}
     \]

3. **Complex Roots**:
   For complex roots, the discriminant \(\Delta = a^2 - 4b < 0\).

   - **Condition for \(|z_1| = |z_2| < 1\)**:
     - Since \( z_1 \) and \( z_2 \) are complex conjugates, \( b = z_1 \cdot z_2 = |z_1|^2 = |z_2|^2 < 1 \).

     Combining these conditions, we get:
     \[
     \begin{cases}
     b \in (0, 1) \\
     b > \frac{a^2}{4}
     \end{cases}
     \]

4. **Geometric Interpretation**:
   - The equation \( b = \frac{a^2}{4} \) represents a parabola \( P \).
   - The equation \( b = a - 1 \) represents a line \( L_1 \).
   - The equation \( b = -a - 1 \) represents a line \( L_2 \).
   - The equation \( b = 0 \) represents a line \( L_3 \).
   - The equation \( b = 1 \) represents a line \( L_4 \).

   The intersections of these curves are:
   - \( P \cap L_1 = L_1 \cap L_4 = \{A(2, 1)\} \)
   - \( P \cap L_2 = L_2 \cap L_4 = \{B(-2, 1)\} \)
   - \( L_1 \cap L_2 = \{C(0, -1)\} \)
   - \( P \cap L_3 = O(0, 0) \)

   The region \( R \) is the union of the regions corresponding to the cases of real and complex roots. It is the interior of the triangle \( ABC \) with vertices:
   \[
   A(2, 1), \quad B(-2, 1), \quad C(0, -1)
   \]

The final answer is the interior of the triangle with vertices \( \boxed{ A(2, 1) } \), \( B(-2, 1) \), and \( C(0, -1) \).