Hi Soles,
Some basic information for you.
Two types of drive. Hub motor (front or rear wheel), or mid drive/crank drive. This fits on the bottom bracket and drives via the chain and gear set.
There are two types of hub motor, geared hub and direct drive. For all sorts of technical reasons best avoid direct drive and consider only geared hubs.
Bikes, legally, can only give assistance when the pedals are being turned. There are two ways of achieving this, torque sensing and cadence sensing also called pas.
Torque sensing takes the power that you put in via the pedals and gives you a multiple power back via the motor. So may give you, say, 10%, 20%, 50%,100% depending on the assist level selected.
Cadence sensing detects the rotation of the pedals and allows the motor to give power, again amount of power depends on level selected.
Most systems these days are 36v or 48volt, either is fine.
Battery capcity/size is measured in Amphours or also quoted in Watt hours. The second method is probably the most useful.
A 14Ah36v battery would give 504Wh. A newby cyclist (still building strength and stamina) may consume around 12Wh/mile. This would give you a notional range of 42 miles (depnds hugely on strength, terrain, wind, rider weight etc).
I would argue that the easier fit is a hub motor and certainly gives a less technical ride style. There are those who say hub motors aren't good at hills. Well, if you look at the bike in my avatar, that's a front hub motor on a carrera. I live in West Wales (where flat bits are illegal) and it's done around 12000 miles over 6 years and I'm on my late 60's.
There's some basics without getting too technical.
Best to say what bike you have to convert. What terrain you'll be riding ie roads or mountains or flat or hilly etc. Your weight (sorry - helps with motor type). And ride style, commute, leisure, touring Etc.