I will be going on holiday soon and I was wondering what happens when you leave the plane. Is it a sense of following the crowd? Do bus services take you to the terminal from the plane? What is the process? Also on a side note is it true that some planes have more than one entrance to them so when boarding the plane such as EasyJet I have seen two doors in which people go in from. Is there a reason for that? Is it just due to seating and stuff?
In general, there are signs and indeed a clear flow of people you can follow. The process is the following:
Get into the terminal building (through a jet bridge connecting the plane to the building, walking on the tarmac or taking a bus). EasyJet planes all have two doors and they use both whenever they can (sometimes it's not possible with a jet-bridge although I have also seen airports where you would get out through a set of mobile stairs and then back inside to reach the middle of the jet bridge). The goal is to load and unload as fast as possible, keeping the plane in the air as much as possible is crucial for them, a plane parked at the airport just costs them money.
Look for the baggage reclaim area. It doesn't matter whether you have hold luggage or not, the way to the exit is almost always through the baggage reclaim area so look for signs mentioning “luggage“ or “baggage”.
Depending on the airport and terminal, you might cross the path of departing passengers, have to walk through some shops or one-way airlocks. Most people coming out of your plane are going there (certainly on EasyJet), you can follow them. Depending on the route and destination, you might also be expected to go through border control before reaching the baggage reclaim so the signs might also say something like “passport control” or mention the name of the country you are entering.
Once you are in the baggage reclaim area, you would be looking for the exit. In Europe, that's also the point at which you have to go through customs so you should see some red or green “nothing to declare” channel and, in some places, a blue channel for EU arrivals. You walk through the appropriate channel and you will usually find a door or lock separating you from the waiting public.
From then on, it varies a lot and it's difficult to give general rules. It's also usually the most confusing part of the process. Leaving the plane and navigating your way to the exit to the “open” part of the airport is typically quite easy.
All that assumes you are not transiting through a third airport on your way to your final destination (but EasyJet only sells point-to-point tickets).
That depends on several things.
In most airports, most planes will drive to a gate after landing, such that when you leave the plane you'll be in the airport, and unless it's a very (you probably have to go to the extreme) small airport there will be signs telling you what way to go when you're arriving. In all the airports I've been in the signs have been in both english and the local language.
If the airport you arrive at is busy (there can be a lot of reasons, they don't really matter here), the plane might park at a "remote" (to the airport building) spot, then you'll leave the plane, go down a set of stairs and into a bus (there will usually be several as they typically don't have the capacity for all the people on the plane) that will drive you to a gate at the airport, at which point you're in the same situation as above.
As the door(s) from the plane are only wide enough for one person, the passengers from each arriving plane will quickly be spread out, so there won't be a big crowd to follow, but if it makes you feel more comfortable you can probably focus on a couple of the people who leaves the plane just before (or after you, the jet bridge leading from the plane into the building should give you plenty of time. But be aware that they might be transiting (i.e. they have to catch another plane, and won't be going the same place as you are) and don't make the feel stalked.
Yes, some planes have more doors used for boarding (and deboarding), that is mostly a question about speed, as mentioned above the doors aren't very wide, so using multiple allows the airline to get people in/out faster. It doesn't really affect the (de)boarding experience.
Edit: Reading what I wrote, I got to think that there's way more small airports than large, and at those the common case might not be that the plane drives to a gate with a jet bridge, so some of my "most"'s shouldn't be taken too literal. Also I forgot to mention that the jet bridges might not fit on smaller planes, so if you're on such a plane and there's room, it might go close enough to the airport that there won't be busses, and you'll have to walk from the plane to the gate, but we're talking very short distances here.
Following the crowd is always a good thing. Although the crowd might not be always going where you are, the statistics are in your favour. There are also plenty of signs, so unless you're departing from/arriving in a really crappy airport, you won't get lost.
As for multiple doors, it is usually on larger airplanes. It helps with the boarding: First/Business class passengers at the front door, economy passengers at the door more or less in the middle.
Usually you are not allowed to "get lost" until you are inside the building, so you would be stopped before wandering of. Inside the building you should be fine unless you pass the "no re-entry beyond this point" which usually is clearly marked. Somewhere between those points should be the baggage claim. And since they want a nice flow in the airport it is usually hard to miss.