As scientifically defined, anything with a cell is considered living. For example, the simplest living forms (unicellular organisms) are bacteria. Bacteria have many of the same capabilities that human cells do. Therefore, they are living organisms just like us. That is why you see sadhus and sadhvis (monks and nuns) covering their mouth while speaking. This is in order to prevent their breath from killing bacteria in the air.
Realistically, we are killing millions of bacteria every day. However, it is important to try to do our best to minimize such violence.
In regard to plants, again, they have an entire cellular network capable of energy production and waste excretion, just like us. Therefore, it is important that we do not purposelessly walk on grass. Playing sports outdoors on grass is acceptable. [Namokar Mantra recitation for the organisms you are harming while stepping on grass is recommended.] However, activities such as cutting through people’s lawns while walking is not necessary and should be avoided in order to refrain from injuring plants and small insects who reside in these lawns.
For consumption purposes, we must sustain ourselves through food. Food can come from two sources: animal or plant. Since plants do not have a nervous system and have limited capability to feel pain, it is more acceptable to eat plants and plant products than it is to use animal products. Ultimately, killing small plants and animals is unavoidable due to our lifestyle. However, decreasing this violence is a priority for all Jains.