Establishing a person's limits when it comes to kink is a vital part of the negotiation/communication process. This is true when starting out with a new partner, but even with existing partners, limits should be regularly reviewed. Within BDSM, the terms hard and soft limist have evolved to reflect that fact that some limits are set in stone, while others may be open to exploration.
Hard Limits Hard limits are things that you would never do under any circumstances. For me, that would include things like scat, bestiality, electrical play, force feeding, cutting my hair, and balloons. When considering what your hard limits are you should consider things that are against your own personal moral code, things that 'ick' you out, and things that might cause health problems for you or trigger your phobias. For example, I suffer from very poor circulation and so cold temperatures can be a big problem for me. Exposure to cold temperatures is one of my hard limits. I also have a huge fear of balloons. This fear has the potential to trigger a panic response. This is important for my Dom/top to know especially if playing in a public/club setting where the presence of balloons as decorations could be possible.
Soft Limits Soft limits are things that are not hard limits in the "no never gonna happen" way. They are either things that at the current time you feel are not for you and/or possibly things that maybe you have no experience with, but may like to explore under the right circumstances. Soft limits are often seen as boundaries and edges that the Dom/top can explore and push with/for you.
As with all aspects of kink, limits are not set in stone. It is completely acceptable for them to change over time. For example, when I first put my soft limits list together, one of the things on there was blood play. I had no experience with it at all. When I thought about it as a concept, it did nothing for me. It neither excited me, nor freaked me out. So, it was a soft limit for me because it was utterly unknown territory. Over time, seeing images and others exploring the kink sparked my interest. Eventually, my Dom decided to push me a little a see what reaction he got. Now, blood play is a huge kink for me.
Likewise, things can also move in the other direction. For example, take face slapping. It was only when it was done to me that I had a huge negative response. It became something that is now a hard limit for me. With that in mind, it is a good idea to regularly review your hard and soft limits with your partner(s) as you evolve to learn more about what does and does not work for you.