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Conditionals are expressions — they return values.

if takes a test, then-branch, optional else-branch:

(if (> x 0)
  "positive"
  "non-positive")

cond for multiple branches:

(cond
  (>= score 90) "A"
  (>= score 80) "B"
  (>= score 70) "C"
  :else "F")

:else is just the keyword :else — it's truthy and serves as the default. You could use true or :default instead; :else is convention.

when is if with no else and an implicit do for multiple statements:

(when (= status "ready")
  (log-it "starting")
  (start-process))

Falsy values: only false and nil. Empty collections, 0, "" are all truthy.