There are 4 characteristics that are altered within weed killers. Pre-emergence is a factor of weed killer that targets seeds that are in the midst of germination. This means that they are best suited to fighting weeds that emerge seasonally due to their reproductive trait. Post-emergence weed killers for grass work more specifically to weeds that have already further developed. Post-emergence usually come in a liquid form that is sprayed on the leaves; whilst pre-emergence is applied to the soil in a liquid form.
Selectivity type is very important as it treats other plants and growth around the weeded area. Selective products will not harm the surrounding plantation or grass (making them more beneficial to gardens and lawn areas). On the other hand, non-selective solutions will effectively damage the roots of all plants in the treated area (more suited to weed growing in drains, pavements and gravel).
Persistence is what the future the state of weed growth will be like. Persistent weed killers will have long term prevention on the growth in the treated area, as germination will be slow but most likely stopped. Therefore this must be considered if you are looking to reseed. If this is the case then the non-persistent short term weed killers for grass will be less harmful to near future planting plans.
Finally there is contact or trans-located. This basically asks how you want the weed to die! Contact herbicides will kill the part of the weed it is applied to, which is perfect for users that want to clear the area of unwanted weeds in areas needed for planting. Trans-located products attack the weed through its internal structure. This process is a little longer but does have more precise and permanent advantage!