Many modern recipes call for a food processor, but that's a luxury that not everyone can afford. If you don't have a food processor, a blender can actually be used in most cases. Since blenders do work differently it's important to know how to transfer the recipe correctly.
A food processor works by using a few blades in a chopping motion. A blender is designed with 2 blades (in most cases) that move around the food. This means there are certain foods that aren't going to work very well in a blender. Soft veggies that need to stay solid will be very difficult to control. If you are making salsa, for instance, the tomatoes will become a mushy mess when placed in a blender, instead of a firm dice. If you are using an ingredient like a tomato, it's best to leave those out of the blender and chop them by hand.
Very firm foods, especially nuts, won't chop in a blender. You can end up damaging the motor trying to get them right, so instead of putting nuts in, consider either purchasing pre-chopped nuts or crush them yourself by hand. You can do this with a rolling pin and a cutting board. Put some elbow grease into it and they will crush without damaging the blades or the motor of your blender.
The blades of a blender also tend to move much faster. It's very easy to liquefy solid foods, so you will need to pay close attention to what you are doing to avoid this.
When you substitute a blender for a food processor, it's important to use the right setting. Some blenders only have 3 speeds, while others may have many more. If you are going to puree food, then the highest setting is okay to use, but if you are trying to create a chopped item, use the slowest setting. You may have to play around a little to find the perfect speed, which the pulse setting works perfectly for. Don't put all of your ingredients in at the first time. This way if it doesn't work out, you haven't wasted everything.
A blender can't handle what a food processor can since it's obviously not designed to function the same. If you are using a blender in lieu of a food processor you will likely need to make several small batches as opposed to a larger, full batch. Start with around 1/4 of the ingredients and work with that first. Once you have it chopped, pasted, or pureed the way you need, you can work on a second batch. This will ensure an even consistency throughout. If you put too much in a blender, the ingredients towards the top of the pitcher won't have the same texture as the bottom, and foods on the bottom will likely end up over-processed trying to chop the top layer.
In a pinch, a savvy cook can use a blender in place of a food processor for some of their ingredients. Some minor adjustments will be needed to make it work for you, but with care, this can be a neat trick to help shoulder some of the work in the kitchen.