Chop cilantro leaves finely. Don't use the stems; they are bitter. Slice onions about 1/8" thick; green and white parts. Break tomatoes if you've used cans of stewed whole tomatoes.
Remove corn kernels from cob. Zap fresh corn in the microwave for 2 minutes with husk; and silk still on. Peel back a bit of husk and test a kernel with your fingermail for doneness. Let cool a bit and remove husks; silk comes off easily. Use knife to cut corn off cob. If there's more than 1/2 c, don't sweat it. Just toss it all in.
If using frozen corn, thaw in a strainer. If you think you'd like more corn, have at it!
Mix well. Best when just made but refrigerates well with almost no loss of quality. If you have any left, that is! Great with chips, of course, but also wonderful on sauteed chicken breasts (saute some onions before you do the chicken; saute chicken until almost done; add salsa and cover; cook until done - - don't overcook it!)