Consider a circle $k$ and a point $K$ in the plane. For any two distinct points $P$ and $Q$ on $k$, denote by $k'$ the circle through $P,Q$ and $K$. The tangent to $k'$ at $K$ meets the line $PQ$ at point $M$. Describe the locus of the points $M$ when $P$ and $Q$ assume all possible positions.
1. **Identify the given elements and their relationships:**
   - Let \( k \) be a circle.
   - Let \( K \) be a point in the plane.
   - For any two distinct points \( P \) and \( Q \) on \( k \), denote by \( k' \) the circle passing through \( P \), \( Q \), and \( K \).
   - The tangent to \( k' \) at \( K \) meets the line \( PQ \) at point \( M \).

2. **Understand the concept of the radical center:**
   - The radical center of three circles is the common point of their radical axes.
   - The radical axis of two circles is the locus of points that have equal power with respect to both circles.

3. **Determine the radical axes:**
   - The radical axis of \( k \) and \( k' \) is the locus of points that have equal power with respect to both circles.
   - The power of a point \( X \) with respect to a circle with center \( O \) and radius \( r \) is given by \( \text{Power}(X) = OX^2 - r^2 \).

4. **Find the radical axis of \( k \) and \( k' \):**
   - Since \( k' \) passes through \( P \) and \( Q \) on \( k \), the radical axis of \( k \) and \( k' \) is the line through \( P \) and \( Q \).

5. **Determine the tangent to \( k' \) at \( K \):**
   - The tangent to \( k' \) at \( K \) is perpendicular to the radius of \( k' \) at \( K \).

6. **Find the intersection point \( M \):**
   - The point \( M \) is the intersection of the tangent to \( k' \) at \( K \) and the line \( PQ \).

7. **Describe the locus of points \( M \):**
   - As \( P \) and \( Q \) vary over all possible positions on \( k \), the line \( PQ \) varies accordingly.
   - The point \( M \) will always lie on the radical axis of \( k \) and \( k' \), which is the line through \( P \) and \( Q \).

8. **Special case when \( K \) coincides with the center of \( k \):**
   - If \( K \) is the center of \( k \), then the tangent to \( k' \) at \( K \) is undefined because \( k' \) degenerates into a line.
   - In this case, the desired locus is an infinite line.

Conclusion:
The locus of the points \( M \) is the radical axis of the circles \( k \) and \( k' \). If \( K \) coincides with the center of \( k \), the desired locus is an infinite line.

The final answer is the radical axis of the circles \( \boxed{ k } \) and \( k' \). If \( K \) coincides with the center of \( k \), the desired locus is an infinite line.