Segunda Clase / Second-Class
There is a wide range in quality for buses classified as second-class. The differences are sometimes regional. My first ride on a Mexican bus was in 1993, on a second-class bus between Oaxaca and Mitla. It was also the worst bus ride I have ever taken. A few days later I took my a second class bus from Morelia to nearby Cuitzeo. I remember thinking, “If this is second-class, then the bus to Mitla was fifth-class.”
 
As first-class buses improve, second-class line sometimes get first-class hand-me-downs as the whole system improves. Some areas, such as central Mexico, the second-classes buses run by companies like Flecha Amarilla and Herraduda de Plata are fairly comfortable and are fine for short trips. Good second-class buses are similar to, and often better than, the Greyhound buses in the US and Canada.
 
Because second-class buses make many stops between stations, they take longer. A second class bus takes an hour and a half or more to cover the same route as a higher class bus does in an hour. There is no reserved seating; board as soon as possible to find a good seat.
 
Most small towns are served only by second-class buses. If you are coming from a long distance, there most likely will not be a direct connection. Take a first class bus to the the nearest large town and make your connect. Those selling tickets will be able to give you your options.
 
Finally, second-class buses are more colorful. While you are not likely to find chickens, you will see a mixture of people. Vendors will come on board usually to sell snacks and drinks, giving the driver a sample. There will be others, reminesent of snake oil salesmen, enthusiastically trying to sell there products. Some will come on at a stop and depart before the bus takes off. Others will ride on the bus for a short time, then get off to catch a bus going the other way as they work a stretch of road. You will travel through small towns rather than than zip down a super highway, adding to your Mexico experience.
 
Primera Clase / First-Class
The main differences between second class and the higher class buses are comfort, speed and the ability to buy a ticket days in advance and to reserve a seat. A light meal is often given in the form of a small sandwich, cookies and a drink. You are given a bag with your goodies as you board. There will probably be a video played. If your destination is nearby, you may get off the bus before the videos ends. The quality of the films is such that you probably won't be missing much. I prefer watching the scenery.
 
De Lujo / Deluxe
The comfort level increases as you move up to deluxe class. First class buses may have an onboard baño, but a deluxe bus definately will.

Ejecutivo / Executive
Extreme comfort is found here, superior to first class seats on an airline. One company, UNO, uses the slogan, “Servcio de avión por tierra (airline service by land).” The restrooms are more comfortable. The seats are widely spaced often with one seat on one side of the aisle and two on the other. Instead of the normal 10 to 12 rows of seats, there may be just 8. So, where they may normally be 40 seats or more on other buses of the same size, an executive bus may have only 24.