Much is unknown about Charlie's appearance. If anyone contests my description of him as against canon (or reasonable deductions from canon), please comment and let me know which part you consider to be incorrect, and what might be better. Cheers!
Defining features: Charlie has the shorter, thicker, sturdier variety of Weasley physique, copious amounts of freckles, presumably blue or brown eyes (to match the known eye colors of his siblings), and red hair, which is presumably kept in at least a semi-long state (his mother attempts to cut it short), though too long would seem to detract from his work with dragons. From this work, we can deduce that he has many burns and other scars. He is a known outdoorsman, a lover of magical creatures, and a superb Quidditch Seeker. Given his noted popularity, many assume him to be handsome; though the only information we know about any sort of intimate relationships is that he never married (source; JK Rowling, not books).