Two moving bodies $M_1,M_2$ are displaced uniformly on two coplanar straight lines. Describe the union of all straight lines $M_1M_2.$
1. **Case 1: Parallel Lines \( l_1 \parallel l_2 \)**

   If the lines \( l_1 \) and \( l_2 \) are parallel, then any line segment \( M_1M_2 \) connecting points \( M_1 \) on \( l_1 \) and \( M_2 \) on \( l_2 \) will be parallel to a fixed direction. This is because the direction vector of \( M_1M_2 \) will be the same regardless of the positions of \( M_1 \) and \( M_2 \) on their respective lines. Therefore, all lines \( M_1M_2 \) will pass through a common point at infinity, implying they are parallel.

2. **Case 2: Intersecting Lines \( l_1 \) and \( l_2 \)**

   Let \( P \) be the intersection point of \( l_1 \) and \( l_2 \). If \( M_1 \) and \( M_2 \) pass through \( P \) simultaneously, then the line \( M_1M_2 \) will always pass through \( P \), making all such lines concurrent at \( P \).

3. **General Case: \( M_1 \) and \( M_2 \) do not pass through \( P \) simultaneously**

   - **Step 1: Define Positions and Miquel Point**

     Let \( A_1 \neq P \) and \( A_2 \neq P \) be the positions of \( M_1 \) and \( M_2 \) at some moment in time. The Miquel point \( M \) of the lines \( l_1, l_2, A_1A_2, M_1M_2 \) is defined. The Miquel point \( M \) is the center of spiral similarity mapping \( A_1A_2 \) to \( M_1M_2 \).

   - **Step 2: Spiral Similarity and Fixed Point**

     Since the ratio \( \frac{A_1M_1}{A_2M_2} \) remains constant, the point \( M \) is fixed. This implies that \( M \) does not change as \( M_1 \) and \( M_2 \) move along \( l_1 \) and \( l_2 \), respectively.

   - **Step 3: Parabola with Focus \( M \)**

     Let \( \Pi \) be the parabola with focus \( M \) that touches the lines \( l_1 \) and \( l_2 \). For any positions of \( M_1 \) and \( M_2 \), the focus \( M \) lies on the circumcircle of triangle \( PM_1M_2 \). Since \( \Pi \) touches \( l_1 \) and \( l_2 \), it must also touch the third side \( M_1M_2 \) of the triangle \( PM_1M_2 \).

     This implies that all lines \( M_1M_2 \) are tangent to the parabola \( \Pi \).

Conclusion:
The union of all straight lines \( M_1M_2 \) either passes through a common point (if \( l_1 \parallel l_2 \) or if \( M_1 \) and \( M_2 \) pass through \( P \) simultaneously) or are tangent to a parabola with focus at the Miquel point \( M \) (if \( M_1 \) and \( M_2 \) do not pass through \( P \) simultaneously).

The final answer is the union of all straight lines \( \boxed{ M_1M_2 } \) either passes through a common point or are tangent to a parabola.