Simply it is an attempt to eat a healthy diet to gain weight, whilst minimising fat gains. The basic principle of bulking to to consume more calories than you expend. Typically a bulking diet will consist of around 6-8 meals per day, spread out every 2-3 hours. A cleverly structured diet will show the breakdown of carbs,protein and fats. Typical bulking calories usually range from aroun 3000 to 6000 and should be tailored to the needs of the individual and specific goals.
Many bodybuilders/individuals will bulk over the winter season and then cut (cutting diet) in the summer. Cutting to cut down on the fat gain by a bulking diet. In general (except n00b gains) it is exceptionally hard to gain weight and loose bodyfat at the same time (excluding the use of steroids+genetics) therefore you cannot bulk and cut at the same time! You can use variety such as cylical bulking e.g. Bulk for 4 weeks/Cut for 2 weeks repeat process..this is an attempt to minimise fat gains.