smc

The line $x+y=1$ is in standard form. Thus, it has a slope of $-\frac{1}{1}=-1$ and a $y$-intercept at $(0,1)$. It is graphed in blue in the diagram. All the points that satisfy this system must lie on this line. The inequality $x^2+y^2<25$ is true for all points strictly less than distance $\sqrt{25}=5$ from the origin. This area is a disk with radius $5$, whose open boundary is represented in red in the diagram. Because the $y$-intercept of the line, $(0,1)$, is inside the disk, it cannot be tangent to the disk. Thus, there must be infiniely many points along the line which are within the disk and thereby satisfy the system. However, because the inequality is strict, the segment of solutions does not include the endpoints, so our answer is $\boxed{\text{a straight line segment not including the end-points}}$.