Though both are units of measure, a ton is a standard measurement of weight equal to 2,000 pounds while a tonne is a metric unit equal to 1,000 kilograms. These are not interchangeable, but the word "ton" has evolved alternate meanings in literature and speech.
When referring to weight, a ton is always a standard amount; however, in expressive writing as well as in opinion writing, this word may be used simply to refer to something weighty or a large amount. For example, "a ton of work" simply means quite a lot of work. In this case, there is no equivalent usage for the word tonne.
From a purely scientific perspective, there is a clear difference between these terms and their meanings. From a less literal view point, the difference becomes even more pronounced based on the concrete usage of one as compared to the other. In addition, one metric tonne is the equivalent of 2,204.62 pounds, so while a ton and a tonne are relatively close in actual weight, the difference is enough to make quite a difference when it comes to nearly everything. Interestingly though, there is no short-hand form for other metric units, such as kiloliters or kilometers.