All airlines have a "minimum" experience level for applicants. Most involve hours logged in flight. There are different categories of pilot time, the most important being "Pilot In Command" time. Basically, if you were a Captain elsewhere or flew single seat planes (general aviation or fighter aircraft) all those hours count. The "biggie" hours are the Pilot In Command (PIC) hours in a pilot's logbook.
Most commuter airlines require between 500-750 hours total time to apply, with the bulk of that being PIC time. Major airlines require something like 2,500 total hours minimum with a minimum of 1,000-1,500 hours PIC. Of course, requirements vary and with what seems like a billion folks on the street after airline downsizing, the airlines can adjust their minimums at will.
Southwest Airlines goes one big step further. Aside from the hourly minimum requirement, we require all applicants to have a type rating in the 737 before starting class. This requires a serious investment of time and money for each applicant. 737 types go for between $8-10,000. And that doesn't guarantee you an interview. What it does help guarantee is commitment to wanting to fly for this Company. It also helps weed out those who aren't up to the task of flying an airliner (which is a lot harder than it looks). We regularly have Top Gun fighter guys sweating through initial training because the type of flying we do here is so very different from what they did in the past. Our civilian applicants are challenged as well. It is a tough course, and their whole future is riding on their performance. You can imagine the pressure they feel knowing the bread on the table of their family at home literally hangs in the balance. After this kind of pressure and stress, navigating around a thunderstorm is more of a nuisance. Once signed-off after training, each applicant is monitored for performance during his or her probationary year and given additional training if necessary.
Aviation ratings are kind of like medical specialties. The Grand Poo-Bah rating is the Airline Transport Pilot (ATP) Rating. This is the pinnacle rating and what every Captain must have to hold his/her position. With competition for jobs, every major airline applicant already has this rating. Think of it as a general "doctoral of medicine" degree. You can practice but you have no specialty.
The type rating is like a medical specialty (urologist, surgeon, neurologist, etc). It certifies the pilot as having shown proficiency under Federal Aviation Administration (FAA) scrutiny to fly a specific airplane as Captain. The oral exams and simulators for the ATP and type rating are virtually identical. In fact, the recurrent Proficiency Check I performed on the 5th of April is virtually the same profile: Instrument work in the weather, engine fires/failures, hydraulic problems --you name it. I have to do this every 12 months here at SWA (in addition to a line check where a Check Pilot rides with me from point A to point B to see how I do with just driving the plane). In fact, for my line check a few weeks ago, I had a Check Pilot watching me and a FAA Safety Inspector watching him and me. It was really cozy up front!