A few key differences come to mind.
1. While students applying to college frequently don't know what they want to study, students applying to grad school have chosen a specific field, so the program search tends to be much more focused.
2. The main essay or personal statement for undergraduate study has a great deal of flexibility and can be written on a huge variety of topics. The statement of purpose for grad school has a fairly specific format and focus, addressing the question of a student's interest in and qualifications for a particular program.
3. Data on undergraduate admissions (e.g., acceptance rates, average GPAs and test scores) are often readily available. This information can be much harder to come by with graduate admissions.