How are airline pilots assigned routes? Are there easy and tough ones?
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.