An interjection used to call someone's attention, similar to 'hey!' or 'oi!'. Casual and somewhat rough.
Hey, come here a sec.
あのどら猫はいつも台所に入り込んでくる。
That stray cat always sneaks into the kitchen.
無精 describes a general lack of diligence or unwillingness to bother with things, whereas どら specifically implies habitual idleness or dissoluteness, often in compounds.
こら is used to scold or reprimand someone sharply, whereas どら is simply a rough call for attention without necessarily implying rebuke.
ぐうたら is a casual, expressive word for someone chronically lazy, similar in register to どら but usable on its own rather than mainly as a prefix.
The origin is uncertain. It may be related to the onomatopoeic sound of a gong (どら) used to call attention, but the connection to the 'lazy' sense is unclear.