Direction tells you most of it, and it takes about thirty seconds to check.
Vertical and Diagonal
A vertical or near-vertical crack in a poured concrete wall is usually shrinkage from the concrete curing, often decades ago. It may leak, and leaking is annoying rather than dangerous. Injection commonly fixes it for $400–$900. See foundation crack repair.
Diagonal cracks running from the corners of windows or from wall corners are usually settlement. Whether they matter depends entirely on whether the settlement has stopped, which is a question about movement rather than appearance.
Horizontal
A horizontal crack running a long way sideways — particularly along a mortar joint in a block wall, somewhere in the middle third of the height — is a different category. That is the wall failing under sideways soil pressure, not water finding a route.
That one is structural, it gets worse rather than better, and it is worth an inspection promptly. The horizontal crack page explains the mechanism.
Stepped
Cracking that steps along the mortar joints in a block wall, like a staircase, is very common in older Kalamazoo and Battle Creek foundations and is frequently harmless. It becomes interesting when it is wide, recent, or accompanied by the wall moving inward.
How to Tell If It Is Moving
Existence is not the question. Movement is.
Mark both ends of the crack with a pencil and the date. Bridge it with a strip of masking tape, or measure the width with a coin against it and photograph that. Check again in a month, and after the spring thaw. A crack that has been the same width for twenty years and one that has opened this spring are different situations, and only one needs money spent on it.
If it is wider than about a quarter inch, or one side sits proud of the other, get it looked at rather than monitored.
Inspections are free — send photographs with something for scale through the contact form.