The files in the/var/cache/apt/archives folder are the installation files that are downloaded when you use the sudo apt-get install appname, which can be cleaned up and, if your system is out of space, you can think of cleaning up from here and getting space. The files under my PC are as follows:
/var/cache/apt/archives $ ls
Acct_6.5.5-1ubuntu4_amd64.deb Libnss3-tools_2%3a3.15.4-0ubuntu0.13.10.1_amd64.deb
Aptdaemon_1.1.1-0ubuntu4_all.deb Libntrack0_016-1.1_amd64.deb
Aptdaemon-data_1.1.1-0ubuntu4_all.deb Libntrack-qt4-1_016-1.1_amd64.deb
At-spi2-core_2.10.1-0ubuntu0.1_amd64.deb Libodbc1_2.2.14p2-5ubuntu4_i386.deb
Avahi-autoipd_0.6.31-2ubuntu4.1_amd64.deb Libogg0_1.3.1-1_i386.deb
Avahi-daemon_0.6.31-2ubuntu4.1_amd64.deb Libopenal1_1%3a1.14-4ubuntu1_i386.deb
Avahi-utils_0.6.31-2ubuntu4.1_amd64.deb Libopencc1_0.4.0-1_amd64.deb
Bind9-host_1%3a9.9.3.dfsg.p2-4ubuntu1.1_amd64.deb Liborc-0.4-0_1%3a0.4.17-2_i386.deb
Bluez-alsa_4.101-0ubuntu8b1_i386.deb Libotf0_0.9.13-1_amd64.deb
Cpp-4.8_4.8.1-10ubuntu9_amd64.deb Libp11-kit0_0.18.3-2ubuntu1_i386.deb
Cups_1.7.0~rc1-0ubuntu5.1_amd64.deb Libpam0g_1.1.3-8ubuntu3_i386.deb
Cups_1.7.0~rc1-0ubuntu5.2_amd64.deb Libpam-gnome-keyring_3.8.2-0ubuntu3.1_amd64.deb
Cups-bsd_1.7.0~rc1-0ubuntu5.1_amd64.deb Libpam-winbind_2%3a3.6.18-1ubuntu3.1_i386.deb
Cups-bsd_1.7.0~rc1-0ubuntu5.2_amd64.deb Libpango-1.0-0_1.32.5-5ubuntu1_i386.deb
Cups-client_1.7.0~rc1-0ubuntu5.1_amd64.deb Libpango1.0-0_1.32.5-5ubuntu1_i386.deb
Cups-client_1.7.0~rc1-0ubuntu5.2_amd64.deb Libpangocairo-1.0-0_1.32.5-5ubuntu1_i386.deb
Cups-common_1.7.0~rc1-0ubuntu5.1_all.deb Libpangoft2-1.0-0_1.32.5-5ubuntu1_i386.deb
Cups-common_1.7.0~rc1-0ubuntu5.2_all.deb Libpangox-1.0-0_0.0.2-4_i386.deb
Cups-daemon_1.7.0~rc1-0ubuntu5.1_amd64.deb Libpangoxft-1.0-0_1.32.5-5ubuntu1_i386.deb
Cups-daemon_1.7.0~rc1-0ubuntu5.2_amd64.deb Libpciaccess0_0.13.2-1_i386.deb
Cups-ppdc_1.7.0~rc1-0ubuntu5.2_amd64.deb Libpcre3_1%3a8.31-2_i386.deb
Cups-server-common_1.7.0~rc1-0ubuntu5.1_all.deb Libphonon4_4%3a4.7.0really4.6.0-0ubuntu2_amd64.deb
Cups-server-common_1.7.0~rc1-0ubuntu5.2_all.deb Libpinyin2_0.8.0-2_amd64.deb
Desktop-file-utils_0.21-1ubuntu3_amd64.deb Libpixman-1-0_0.30.2-1ubuntu0.1_amd64.deb
Dnsutils_1%3a9.9.3.dfsg.p2-4ubuntu1.1_amd64.deb Libpixman-1-0_0.30.2-1ubuntu0.1_i386.deb
Docbook-xsl_1.78.1+dfsg-1_all.deb Libplasma3_4%3a4.11.3-0ubuntu0.1_amd64.deb
Duplicity_0.6.21-0ubuntu4.1_amd64.deb Libplasma3_4%3a4.11.5-0ubuntu0.1_amd64.deb
Enigmail_2%3a1.5.2-0ubuntu1_amd64.deb Libpng12-0_1.2.49-4ubuntu1_i386.deb
Eog_3.8.2-1ubuntu1.1_amd64.deb Libpolkit-qt-1-1_0.103.0-1ubuntu1_amd64.deb
Esound-common_0.2.41-11_all.deb Libpoppler-qt4-4_0.24.1-0ubuntu1_amd64.deb
Espeak-data_1.47.11-1ubuntu0.1_amd64.deb Libpostproc52_6%3a0.8.10-0ubuntu0.13.10.1_amd64.deb
Fbterm_1.7-2_amd64.deb Libprocps0_1%3a3.3.3-2ubuntu9_amd64.deb
Fcitx_1%3a4.2.8.1-3_all.deb Libproxy1_0.4.11-0ubuntu4_i386.deb
Fcitx-anthy_0.2.0-1_amd64.deb Libpulse0_1%3a4.0-0ubuntu6_i386.deb
Fcitx-bin_1%3a4.2.8.1-3_amd64.deb Libpulsedsp_1%3a4.0-0ubuntu6_i386.deb
Fcitx-chewing_0.2.1-1_amd64.deb Libpulse-mainloop-glib0_1%3a4.0-0ubuntu6_i386.deb
Fcitx-config-common_0.4.7-1_all.deb Libpurple0_1%3a2.10.7-0ubuntu4.1.13.10.1_amd64.deb
Fcitx-config-gtk_0.4.7-1_amd64.deb Libpurple-bin_1%3a2.10.7-0ubuntu4.1.13.10.1_all.deb
Fcitx-config-gtk2_0.4.7-1_amd64.deb Libqapt2_2.0.65-0ubuntu1_amd64.deb
Fcitx-data_1%3a4.2.8.1-3_all.deb Libqapt2-runtime_2.0.65-0ubuntu1_amd64.deb
Fcitx-dbg_1%3a4.2.8.1-3_amd64.deb Libqca2_2.0.3-4_amd64.deb
Fcitx-frontend-all_1%3a4.2.8.1-3_all.deb Libqt4-dbus_4%3a4.8.4+dfsg-0ubuntu18.1_amd64.deb
Fcitx-frontend-fbterm_0.2.0-2_amd64.deb Libqt4-dbus_4%3a4.8.4+dfsg-0ubuntu18.1_i386.deb
Fcitx-frontend-gtk2_1%3a4.2.8.1-3_amd64.deb Libqt4-declarative_4%3a4.8.4+dfsg-0ubuntu18.1_amd64.deb
Fcitx-frontend-gtk3_1%3a4.2.8.1-3_amd64.deb Libqt4-declarative_4%3a4.8.4+dfsg-0ubuntu18.1_i386.deb
Fcitx-frontend-qt4_1%3a4.2.8.1-3_amd64.deb Libqt4-designer_4%3a4.8.4+dfsg-0ubuntu18.1_amd64.deb
Fcitx-frontend-qt5_0.1.1-1_amd64.deb Libqt4-designer_4%3a4.8.4+dfsg-0ubuntu18.1_i386.deb
Fcitx-googlepinyin_0.1.6-1_amd64.deb Libqt4-help_4%3a4.8.4+dfsg-0ubuntu18.1_amd64.deb
Fcitx-hangul_0.3.0-1_amd64.deb Libqt4-network_4%3a4.8.4+dfsg-0ubuntu18.1_amd64.deb
The cleaning method is simple:
/var/cache/apt/archives $ sudo apt-get clean
The *.deb file does not exist when you view it again.
/var/cache/apt/archives $/var/cache/apt/archives$ LL
Total 28
Drwxr-xr-x 3 root 20480 February 28 20:04./
Drwxr-xr-x 3 root 4096 February 28 20:04. /
-rw-r-----1 Root 0 January 06:54 lock
Drwxr-xr-x 2 root root 4096 February 19:45 partial/
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