Plant orders are very technical, because as others have said, differentiation in plants has to do with primarily reproductive structures and arrangements, also stem, leaf, and root anatomy. Here’s a few to give you an example:
Inflorescence a spadix [dense spike] with spathe; flowers weakly monosymmetric, tapetal cells 2-4-nucleate, pollen endexine spongy, carpels ascidiate-plicate, ovules atropous; endosperm cellular [first division transverse, divisions in each domain similar], copious; collar rhizoids +.
- POALES (Grasses and grass-like plants)
Mycorrhizae absent; vessels also in stem and leaf; SiO2 epidermal; raphides 0; septal nectaries 0, styles separate, or single and strongly branched, dry; endosperm nuclear, embryo broad, short to minute; mitochondrial sdh3 gene lost.
- ARECALES (Palms and close relatives)
Plant woody, usu. monopodial; vessels also in stem and leaf; cuticular waxes as aggregated rodlets, stomata tetracytic; leaves spiral, petiolate, plicate, pinnately (palmately) pseudocompound or deeply divided; septal nectaries +, 1 apotropous ovule/carpel; embryo short, broad.
- ROSALES (Roses and close relatives)
Dihydroflavonols +; roots diarch [lateral roots 4-ranked]; prismatic crystals in ray cells [not Barbeyaceae, Elaeagnaceae]; (sieve tubes with non-dispersive protein bodies; sieve tube plastids lacking starch [Rhamnaceae, Dirachmaceae?]); mucilage cells +; stomata anomocytic; leaf margins with teeth; inflorescence cymose; hypanthium +, nectariferous, K valvate, C clawed, 1 apotropous ovule/carpel, micropyle endostomal, styles +, stigma dry; K and/or hypanthium persistent in fruit; (polyembryony +).
Within plants, appearance is so diverse that you can’t necessarily give general appearance without being specific, because many plants mimic others.