Here are some key features of these early computers:
Mark I:
Electromechanical computer built in 1944
Used relay switches and rotors for computation
Could do addition, subtraction, multiplication and division
Programmed using punch cards and paper tape
Very slow, about 3-5 seconds per calculation
ABC:
First fully electronic computer, designed in 1937
Used vacuum tubes instead of mechanical parts
Could solve systems of linear equations
Programmed by rewiring the circuits
Not a stored program computer
ENIAC:
Built in 1946 for calculating artillery trajectories
Contained 17,468 vacuum tubes
Programmed by plugging cables and setting switches
Could do 5,000 additions/subtractions per second
Limited programmability and storage
UNIVAC I:
First commercial computer produced in 1951
Used vacuum tubes and magnetic tape for memory
Stored program computer with compiler
About 1,000 times faster than Mark I
Weighed 16,000 pounds and used 5,600 vacuum tubes
Used for applications like census data processing
In summary, these moved from electromechanical to electronic designs, with increasing speed, programmability and memory storage capacity over time.