Just that really. My son's 18 months old and although he does a lot of babbling, he only says 2 or 3 recognisable words (i.e. 'Dee' for 'birdie' and 'Dar' for 'Star'), and doesn't seem that fussed about speaking.
He's definitely understanding things - if I ask him to sit down he'll (sometimes) do it. If I ask him to go get the ball/bear/doll/etc he will happily oblige. He loves to sit on my knee and read the 'That's not my...' books and he'll point at the mouse on each page, etc. I got him a book of 100 first words and he'll point at a thing so I say what it is, but there's no repeating.
He's my only child, so it's not a case of an older sibling talking for him. I'm mostly not worried, I know kids all develop differently, and he's not obviously behind on any of the other 'normal' milestones. I'm just wondering if there's anything I can do to help/encourage his speech.