Start-ups here I refer to high-end tech start-ups. There interview process is really fantastic. If you are a fresher going for an interview for a start-up, you need to have a very good understandings of computer science fundamentals like Data Structure, Algorithms, Operating Systems, Data bases, and basics of networking.
Start ups really look for smart people, because what I feel is that no one starts a company just to check whether it works or not, they usually start with a vision/mission (what ever you call it) and they want good people to join them in accomplishing it.
Coming to their interview process they ask you real good algorithmic questions, its not just asking you whether you know the Matrix multiplication dynamic programming problem, but ask you a question where in you need to apply the same. I came across two cases one in which they test how (broad) many various technologies you have touch with and in the other case they test what is the technology in which you are exceptional.
Usually in start-ups you have a problem to solve and you also know a perfect solution which is very difficult to explain to others or implement. In this case you need to come up with a way/method to solve the problem irrespective of the order of complexity/ running time. Let’s launch the product, then we can come back and make it more efficient 😛 or spend good amount of time to implement it efficiently and then go out. Your choice 😉 . For example lets consider this, I think you should be aware of balancing a chemical equation, this might be done constructing a matrix and solving it using a Gaussian method or simplex method or some other stuff which are very hard to implement. This might be a efficient solution which is hard to implement. You can probably come up with a trail and error approach to balance the equation, which is a very dumb way of doing it. This is just a random example which struck my mind 😛
Note: This post is not at all a comparison between the start-ups and BIG companies :-). BIG ones might be/are much more harder.