Those are card reader slots.

Various types of memory cards can be used.

A memory card, (Or can be stated as Flash Memory Card), takes the place of the old Floppy Disks, for one.

It is digital. There is no disk inside, just hardware electronic components.

Due to the size of trying to explain every type of memory card that is available, and the memory card itself, I would like to give you this link,

http://en.wikipedia.org/wiki/Memory_card

Here it explains the basic format of a memory card.
You can click on the titles of the memory card/s, (in blue), to get a deeper explanation, of that particular memory card.

The specifications for your eMachines ET1810 Desktop PC, state that you have the following card reader slots for,

1.Compact Flash (Type I and II)
2.CF Microdrive
3.MultiMedia Card (MMC)
4.MMCmobile
5.MMCmicro
6.Reduced-Size MultiMedia Card (RS-MMC)
7.Secure Digital Card (SD)
8.miniSD Card
9.xD-Picture Card
10.Memory Stick
11.Memory Stick PRO
12.Memory Stick PRO Duo
13.Memory Stick Micro (M2)

Look above each slot for an abbreviation.
For example on my computer,

I have one card reader slot that is approximately 1 inch wide.
It looks like a T that is stretched out.
It has SD/MMC/SM/MS/MS-PRO, above it.

This means that the card reader slot will support, (Use),
[In order as printed above],

1.SD Card - Secure Digital Card
2.MultiMedia Card.
3.Smart Media card
4.Memory Stick
5.Memory Stick Pro

I have one more card reader slot. It has CF-Microdrive above it. A wider slot than the above, and looks like a upside down stretched out T.

This means of course that it uses, (Supports), a Compact Flash Microdrive card

I've tried to explain the various card reader slots, on your computer's front panel, without being too technical, or being offensive in nature.

If you want, I can explain as technical as you wish, or simplify my solution more.

This is the Specifications page for your eMachines ET1810,

http://support.gateway.com/emachines/desktop/2009/emachines/et/et1810/ET1810sp2.shtml

Scroll down, and look to the right of the heading - Card Reader.
These are the cards your computer supports, (Uses)

Looking at the main support page for your eMachines, it looks as though you have four card reader slots.
Three grouped together on the left side, and two in the middle.

The two slots to the far right are USB ports, with probably a headphone jack port, and a microphone port to the right of them.

This page gives a little more detail, as to what your card reader slots support,

http://support.gateway.com/emachines/desktop/2009/emachines/et/et1810/ET1810fv.shtml