The Quartier Latin and surroundings can be divided in four sections :

• the quartier latin itself : The quartier latin takes it's name from other similar neighbourhoods in other cities like Paris where students use to learn, way back then, in latin and speak it in the streets. The old schools that were in Montreal's quartier latin at the turn of the century (Université de Montréal, the École Polytechnique, etc) have been demolished or transformed but the Université du Québec à Montréal (UQAM), built in the 1970's in that district, justifies its name once again.
  Mostly centered around St-Denis street between Ste-Catherine and Sherbrooke street near the UQAM's main campus, it's where you will find most cafes (Le St-Sulpice, Café Chaos, etc), restaurants (Le Commensal, Formosa, etc), the St-Denis theater, and for movie buffs the huge Quartier latin movie theatre, the National Film Board of Canada and Cinémathèque Québécoise, etc. There are many francophone university and college students around so the crowd at the restaurants and bars is usually young but there are also "older" people going to the St-Denis theater or to the various restaurants. Most of the cafes and bars are great places to hang around with friends (no one goes there for dancing or for pick-up bars... this isn't Crescent street after all). There's also Ontario street near St-Denis that has interesting restaurants (La Paryse, Chez Gatse, etc) and pubs (L'Ile Noire).
  The latin quarter's merchant association has its own web site with more information : www.quartierlatin.ca (in french only).

• the "red light" : it is nothing like it was before (from the 1920's to the mid-50's) when it was much larger and there were hundreds of bordellos, gambling houses, bars and clubs. The red-light was known throughout north america as the place to be (especially during the prohibition when the province of Quebec was the only place in north america where it was legal to produce and sell alcohol). It is now much tamer and smaller, it starts from Clark street to the west and going to Berri street to the east, from de Maisonneuve blvd. to the north and René-Levesque blvd. to the south. There are stip clubs, sex shops, arcades and peep shows, bars and clubs mostly around the now famous corner of Ste-Catherine street and St-Laurent blvd and there's a lot of interesting characters around. There's a very diverse crowd and some prostitutes (around Ste-Catherine and St-Laurent or St-Denis and René-Levesque) but it is not dangerous at all at any time of the day or night. P.S. the section of the "red light" between Ste-Catherine and René-Levesque is mostly large parking lots and some houses, nothing really interesting there. The most famous bars and clubs are the Foufounes Electrique, a punk and alternative hangout, and the Dome, a large dance club. As for St-Laurent blvd., the section between René-Levesque to the south and Ste-Catherine street to the north is where you will find the Monument National theater, a couple of greasy spoons including the Montreal pool room and arcades, peep shows and clubs. There are a couple of army surplus stores, used clothes stores as well as inexpensive electronics stores (for small things like radios, calculators, etc) in the section north of Ste-Catherine but south of Sherbrooke.

• the gay village : the village is actually not part of the quartier latin either (I had to put it somewhere) but it has also a good reputation for its bars and clubs. The village is part of the centre-sud district of Montreal (aka south-central aka Faubourg àla m'lasse aka Ste-Marie) located to the east of the "red light" (between Berri street to the east and Papineau to the west and Sherbrooke to the north and Notre-Dame to the south). The main streets are Ste-Catherine east of Beaudry street, where most gay bars, dance clubs, stores and restaurants are located and Ontario where there are some cafes and restaurants. Actually, this area has gone through and important revitalization the past few years thanks to all those new restaurants, stores and clubs on Ste-Catherine street east and Ontario street. If you haven't been there for a while, it is worth another visit. For more information, you can check out the tourist information center for the gay village.

• Carré St-Louis and the St-Laurent blvd. north of Sherbrooke : technically part of the Plateau Mont-Royal, the Carré St-Louis and St-Laurent blvd.north of Sherbrooke street but south of Des Pins ave. is where there are a dozen or more excellent restaurants and Prince-Arthur street, a pedestrian street with a lots of - mostly touristy - restaurants and - not touristy - bars. It's also on Prince-Arthur that you'll find street performers and people who'll draw your caricature (if you ever wondered what you would look like if you had a big head and skis, this is the place for you... seriously, it's not that bad). As always, St-Laurent blvd. can be considered a district in itself, the section between Sherbrooke and des Pins ave. is where you will find the highest concentration of trendy restaurants in Montreal (there's at least a dozen of them in that section) and there's a couple of bars and clubs including Angel's and Jaï as well as two great pool places (Bacci and the Swimming in the afternoon). This last section is the most busy at night with lot's of people walking by and it's one of the best place in the city for a night out.

Surrounding streets :

Clark street to the west

Berri street to the east (the gay village is between Berri and Papineau streets)

des Pins ave. to the north

René-Lévesque blvd to the south.

Metro stations :

St-Laurent

Berri-UQAM

Sherbrooke

Beaudry