How the body reacts to certain foods is what determines what foods are alkaline for the body and what foods are acidic. For example lemons and limes are acidic in nature, yet they have an alkalizing effect on the body once they are digested.
Acidic vs. alkaline foods
Nearly all fruits, vegetables, herbs, nuts, and seeds have an alkalizing effect on the body, though their degree of alkalinity can vary. Some (not all) of those same foods have an acidic effect on the body once they are processed into oils and nut butters. A few grains such as quinoa and wild rice are considered alkaline foods, and nearly all tea, except black tea, is alkaline.
On the opposite end of the spectrum, nearly all animal proteins (meat, poultry, fish, eggs, and dairy) have an acidic effect on the body. Before vegetarians rejoice, most beans and legumes --- the core vegetarian protein --- also make the list of acidic foods. Most grains, certain fats and oils, and a few vegetables also make the list. Processed foods as an entire category are also considered acidic.
What are alkaline foods?
In general, foods that are not processed and that are closest to their natural state tend to be alkaline. Most fast food, packaged foods, and processed grains (white rice, white flour, etc.) tend to be acidic. Pesticides tend to be acid-forming, so organic fruits and vegetables are best for an alkaline diet.
It is important to understand the benefits of eating alkaline foods, but to note that we should not eliminate acidic foods altogether. Instead, we should balance our diet by eating 60-80% alkaline foods and 20-40% acidic foods.
Here is a chart that shows what foods are alkaline or acidic by degree (very low to very high). And another chart makes it easy to see alkaline forming foods and acid forming foods by type.