find_cycle and simple_cycles are two functions provided by the networkx library in Python for finding cycles in a graph.

find_cycle is a function that finds a cycle in a directed graph. It returns a list of nodes that form a cycle. If no cycle is found, it returns None. The function uses a depth-first search algorithm to find a cycle. If there are multiple cycles in the graph, it returns one of them.

simple_cycles is a function that finds all simple cycles in a directed graph. It returns a list of lists of nodes, where each inner list represents a simple cycle. If no cycle is found, it returns an empty list. The function uses an algorithm based on Johnson's algorithm to find all simple cycles.

The main difference between the two functions is that find_cycle returns only one cycle, while simple_cycles returns all simple cycles in the graph. Additionally, find_cycle is faster than simple_cycles for finding a single cycle, while simple_cycles is better suited for finding all cycles in a graph.