How are routes given to airline pilots? Is it based on the amount of flying experience they have or just random or based on availability?
Also, can routes be classified into tough and easy? If so, what are some examples? In not, then are all routes equal? Doesn't more traffic in a particular area correspond to a harder route?
The answers to your questions vary greatly depending on the individual airline, type of equipment, what kind of flying they do, as well as contractual and union (if any) work rules. The one thing that is generally the same and as noted in the previous answer and comment, is that scheduled lines of flying are published to the crews, and a bidding system based on seniority is used to assign those lines.
Also, can routes be classified into tough and easy?
What one pilot considers tough another may consider easy. A better way perhaps to think of it is that what one pilot considers desirable, another may not, and that will be reflected in how the individual pilots bid.
Some examples (but certainly not exhaustive): Young pilots with families would typically bid away from Christmas flying. I, however, not being a fan of Christmas and with all my children grown, deliberately bid to work on Christmas to open a no-work-on-Christmas line for someone else. In the northern hemisphere winter, I would bid into Australia, South America, or Africa because I don't like the short days of winter. In the northern hemisphere summer I tended to bid into places where I stood a good chance of getting to a beach. A lot of pilots bid simply to maximize income. And there were those who bid to maximize overnights at a place where they had an ongoing personal relationship.
Whether or not a pilot gets to actually fly a line as published is another matter. If you're flying for a scheduled airline that does no on-demand charter work, and you're going from one company station to another company station, chances are good that you'll get to fly the line as published. If, however, you're flying for a company that does on-demand charter work, the probability of flying the line as published goes down as the percentage of charter work goes up, and this is especially true with a mixed fleet of passenger and cargo aircraft. I flew for two commuter airlines, and I almost always flew my awarded bid line as published. Then I flew for two 747 carriers, both of which did a lot of on-demand charters. In the 12 years of flying for the 747 carriers, I rarely flew a line exactly as it was published. If was a matter of when you would have to depart from your line, not if.
There are companies that do nothing but on-demand charters. In those cases what you're bidding is not where you're going to fly, but which days you will be on-call.
Doesn't more traffic in a particular area mean more tougher?
Some would consider it tougher, some more fun. A higher workload certainly, but I always felt that was more fun. There's something addictive, at least for me, to running approaches in high-density traffic. It's quite exhilarating really.
Most airlines use a Preferential Bidding System to determine routes. Basically, pilots submit which routes they want to fly (based on location, schedule, etc.) and then a system assigns them routes, with more senior pilots having preference on getting their choices.
I will try to add an answer which is more directed towards the second part of your question, "are there easy and tough ones".
Just like there are certain airports with rather difficult approach and landing procedures which may require special instruction and familiarization of the pilots (some are even further restricted such that only the captain is allow to do the landing), the same is true for particular routes (i.e. airways) which are simply more challenging than others.
One example is 'L888' (sometimes called 'the silk route'), an important short-cut especially for some European carriers on the way to Hongkong or Guangzhou. It runs straight over the Tian Shan mountain range, north of the Himalaya, with very high Minimum Safe Altitudes in some parts. In case of an emergency forcing an immediate descent (like loss of cabin pressure) or a drift down (engine failure), there are only very few and spatially constrained escape corridors that allow a safe descent and diversion towards lower terrain. Therefore, flights on L888 require extensive planning both before and throughout the flight, and usually airlines will make sure that the crew has received a specific briefing and/or training for this sector.
Another example would be routes crossing the Intertropic Convergence Zone (ITC) e.g. on flights from Europe to South America, where especially challenging weather phenomena may occur.
Regarding the question whether more experienced crew will get those routes: at least for one major European carrier I am aware of, this is sort of true in the sense that all their crew will initially start out flying short-haul aircraft, and only after some years of experience they are promoted to long-haul and then exposed to these routes (of course, there may also be challenging short-haul sectors, requiring e.g. prior airport familiarization).
So, to summarize, yes, there are airports and routes which are more challenging than others, and both airlines and regulations will make sure that only pilots who are sufficiently qualified will get these routes. The crew scheduling departments will usually know which pilots have received said instruction courses and plan accordingly.
