Anything that becomes popular. Trends come and go. For example the skinny jeans movement. It became popular and everyone was wearing it and before that it was baggy clothes. This applies to everything not just clothes.
The word "trend" can be defined as the direction "something" seems to be going. This "something" can be animate, such as a flock of geese moving generally northward during spring in the northern hemisphere. Or the "something" can be inanimate, such as a rock slide or an automobile rolling to a stop. And then the "something" can be sociological, such as the direction a political election is leaning or the way education is changing.
All of these point out that a trend is not absolute, but the odds are in favor of "something" moving in a particular direction.
Adding the -ing ending to the word "trend" emphasizes the process or movement aspect of a trend. A trend may point to a probable end, but trending deals with the process of getting there.
If you are referring to a trending topic on Twitter or another site like that, it just means that a certain tag (ex : #RebeccaBlack) is being used by a lot of people and is therefore a "trend". They change on a daily basis.