Simply put, it is a league-cum-knockout championship of the lower-ranked teams of the first division tiers in Europe. The number of qualifiers depends on how well the team finishes in the first division leagues of their countries. It also depends on the ranking of the country as per UEFA rating scores. E.g. The English Premier League, La Liga, Bundesliga, etc. have three spots which are given to the teams finishing 5,6, and 7 respectively while the Belgium Pro league has two spots.
After the first league round of twelve groups, the top two teams qualify for the second round (round of 32). Here, 8 third-placed teams from the first league round of the UEFA Champions League will join the 24 qualifiers. The second round is a two-legged round where 16 winners will be determined. This process goes on till the final. The final is a single-legged match played on a predecided venue. The 2017 Europa league will happen in Friends Arena in Stockholm.