VBA: Difference between & and +

Dim str1 As String
Dim str2 As String

str1 = "Hello"
str2 = "World"

Dim combinedString As String
combinedString = str1 & " " & str2

Dim num1 As Integer
Dim num2 As Integer

num1 = 5
num2 = 10

Dim sum As Integer
sum = num1 + num2