For thirty years the Champions League ran on four-team groups. Two clubs advanced, two did not, and by the fifth matchday a large share of fixtures no longer mattered. The competition solved that by abolishing groups entirely and putting every qualified club into one league table.
The practical effect is that the competition now has a mid-season standings page worth reading. A club sitting eleventh in November is not safe and not out. It is in the range where one result moves it between a direct place in the round of 16 and an extra two-legged tie in February, and that distinction is worth a great deal in a crowded calendar.
The other change is who you play. Instead of three opponents twice, a club plays eight different opponents once each. Nobody gets a favourable group and nobody gets a group of death, because there are no groups. What remains is a seeded draw that guarantees a mix of difficulty, and a table where goal difference across eight matches genuinely decides places.
The knockout half is unchanged and needs no explanation: two legs, aggregate score, extra time, penalties, then a single final at a neutral stadium. Away goals no longer apply, which has made second legs more cautious and extra time more common.
Qualification for the competition itself is decided in domestic leagues the previous season, and the number of places a country receives depends on its clubs' European results over a rolling five-year period. That is why a strong season by mid-table clubs in the Europa League can hand their league an extra Champions League place two years later — a slow-moving mechanism most fans only notice when it benefits them.
There is also a route in for clubs who finish outside the qualifying places: the Europa League winner enters automatically. It is the one door in European football that opens regardless of league position, and it turns a secondary competition into a genuine season objective for clubs whose domestic form has collapsed.
For anyone watching casually, the practical guide is short. Between September and January, watch the single table and the line dividing the top eight from the rest, because that line is the difference between a February off and two extra knockout matches. From February, it is a straight bracket, and the only thing that matters is who survives the second leg.