A graph is a set of objects plus the relationships between them. The objects become nodes, and each relationship becomes an edge joining two of them. Whenever a problem is about connections, a graph is the natural fit.
Real-world examples include:
Road maps, where intersections connect by streets
Social networks, where people connect by friendship
The web, where pages connect by hyperlinks
Task schedules, where jobs connect by "must run before"
The payoff is that the same algorithms work on all of these. Questions like "can this node reach that one" or "what is the cheapest route" become the same search over any of them. That reuse is why graphs show up in routing, package managers and recommendation systems.
Rewriting in plainer words…
This answer doesn't lend itself to a diagram - it reads best . No credits were charged.
Is a tree a graph? What separates the two?A tree is a connected graph with no cycles, so n nodes have n-1 edges. A general graph can have cycles or disconnected parts.
What you can say
A graph is a set of objects and the relationships between them. Each object is a node, and each relationship is an edge joining two nodes.
I reach for one whenever a problem is really about how things connect.
A road map is one, with intersections joined by streets, and a social network is another, with people joined by friendship.
The web joins pages by hyperlinks, and a task schedule joins jobs by which one must run before another.
The payoff is reuse, because 'can this node reach that one' and 'what is the cheapest route' turn into the same search on any of them.
That's why graphs show up in routing, package managers and recommendations, though if your data has no links worth asking about, a plain table or list is simpler.
Weak answers to avoid
Stops at nodes and edgesNodes plus edges is the textbook line. Add what graphs model and the payoff, that one reachability or shortest-path search works on every one of those problems.
Talks about charts and plotsHere a graph means nodes and edges, not a bar chart or an x-y plot. Mixing the two up in the first sentence shows a gap in CS basics.
Lists examples with no common threadReciting roads, friends and links without saying why they belong together misses the point. They share algorithms, so the same search code runs on each.
Assumes every edge goes both waysHyperlinks and 'must run before' only go one way. Ignoring direction gives wrong reachability answers, so say which examples are directed.
Interview lens
Likely follow-ups, what you can say, and the weak answers to avoid.
