Short answer: Conditional = buy IF conditions met (finance, inspection). Unconditional = must buy, no backing out.
A conditional offer means you will buy if certain things happen — like getting finance, a good building report, or selling your current home. An unconditional offer means you must buy no matter what. No backing out. Unconditional is stronger to sellers but riskier for you. First home buyers should usually make conditional offers.
Conditional Offer: $600,000 subject to finance and inspection
Unconditional Offer: $600,000, no conditions
Seller Choice: Unconditional wins if prices are equal
Buyer Risk: Unconditional = no protection if house has problems