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List literals look like Python:

def nums = [3, 1, 4, 1, 5, 9]
nums[0]                 // 3
nums.size()             // 6
nums << 2               // append (mutates)
nums + [10, 20]         // new list
nums.sort()             // mutates and returns sorted

Map literals use colons (NOT =>):

def person = [name: "Ada", age: 36]
person.name             // "Ada"
person["name"]          // also works
person.role = "CEO"     // add
person.each { k, v ->
    println "$k: $v"
}

Map keys are strings by default. To use other types, quote with parens: [(1): "one"].

Groovy's collection API is rich:

nums.find { it > 3 }            // first matching
nums.findAll { it > 3 }         // all matching
nums.collect { it * 2 }         // map
nums.sum()                      // 23
nums.unique()                   // distinct

it is the implicit single parameter inside a closure.